Make gdb/regcache.h self-contained.

Make gdb/regcache.h self-contained.

	gdb/
	* regcache.h (struct regset): Declare.

Commit 0b3092721e5cfa1697f1dafe81efefdbb0236f21 added uses of struct regset to
gdb/regcache.h, but that struct is not declared in this file, and, as it
happens, also nowhere else in the #include chain on x86 GNU/Hurd.  This results
in warnings/errors such as:

    gcc-4.8 [...] ../../W._C._Handy/gdb/gdb.c
    In file included from ./nm.h:25:0,
                     from ../../W._C._Handy/gdb/defs.h:454,
                     from ../../W._C._Handy/gdb/gdb.c:19:
    ../../W._C._Handy/gdb/regcache.h:190:9: warning: 'struct regset' declared inside parameter list [enabled by default]
             size_t size);
             ^
    ../../W._C._Handy/gdb/regcache.h:190:9: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
    ../../W._C._Handy/gdb/regcache.h:193:10: warning: 'struct regset' declared inside parameter list [enabled by default]
              int regnum, void *buf, size_t size);
              ^