[gdb, hurd] Repair build after "Convert struct target_ops to C++" changes

[gdb, hurd] Repair build after "Convert struct target_ops to C++" changes

..., that is commit f6ac5f3d63e03a81c4ff3749aba234961cc9090e causing:

    In file included from [...]/gdb/gnu-nat.c:24:0:
    [...]/gdb/gnu-nat.h:123:1: error: expected class-name before '{' token
     {
     ^
    [...]/gdb/gnu-nat.h:128:16: error: 'inferior' has not been declared
       void detach (inferior *, int) override;
                    ^~~~~~~~
    [...]/gdb/gnu-nat.h:132:8: error: use of enum 'target_xfer_status' without previous declaration
       enum target_xfer_status xfer_partial (enum target_object object,
            ^~~~~~~~~~~~~~~~~~
    [...]/gdb/gnu-nat.h:132:46: error: use of enum 'target_object' without previous declaration
       enum target_xfer_status xfer_partial (enum target_object object,
                                                  ^~~~~~~~~~~~~
    [...]/gdb/gnu-nat.h:124:8: error: 'void gnu_nat_target::attach(const char*, int)' marked 'override', but does not override
       void attach (const char *, int) override;
            ^~~~~~
    [...]

    [...]/gdb/gnu-nat.c: In member function 'virtual void gnu_nat_target::detach(inferior*, int)':
    [...]/gdb/gnu-nat.c:2286:34: error: 'ops' was not declared in this scope
       inf_child_maybe_unpush_target (ops);
                                      ^~~
    [...]/gdb/gnu-nat.c:2286:34: note: suggested alternative: 'open'
       inf_child_maybe_unpush_target (ops);
                                      ^~~
                                      open
    [...]/gdb/gnu-nat.c:2286:3: error: 'inf_child_maybe_unpush_target' was not declared in this scope
       inf_child_maybe_unpush_target (ops);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    [...]/gdb/gnu-nat.c:2286:3: note: suggested alternative: 'maybe_unpush_target'
       inf_child_maybe_unpush_target (ops);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       maybe_unpush_target

    [...]/gdb/i386-gnu-nat.c:200:1: warning: 'void gnu_store_registers(target_ops*, regcache*, int)' defined but not used [-Wunused-function]
     gnu_store_registers (struct target_ops *ops,
     ^~~~~~~~~~~~~~~~~~~
    [...]/gdb/i386-gnu-nat.c:109:1: warning: 'void gnu_fetch_registers(target_ops*, regcache*, int)' defined but not used [-Wunused-function]
     gnu_fetch_registers (struct target_ops *ops,
     ^~~~~~~~~~~~~~~~~~~
    [...]
    /usr/bin/ld: i386-gnu-nat.o:(.data.rel+0x0): undefined reference to `vtable for i386_gnu_nat_target'
    collect2: error: ld returned 1 exit status

	gdb/
	* gnu-nat.c (gnu_nat_target::detach): Instead of
	'inf_child_maybe_unpush_target (ops)' call 'maybe_unpush_target'.
	* gnu-nat.h: #include "inf-child.h".
	* i386-gnu-nat.c (gnu_fetch_registers): Rename/move to
	'i386_gnu_nat_target::fetch_registers'.
	(gnu_store_registers): Rename/move to
	'i386_gnu_nat_target::store_registers'.