[gdb, hurd] Repair build after "Share fork_inferior et al with gdbserver" changes

[gdb, hurd] Repair build after "Share fork_inferior et al with gdbserver" changes

..., that is commit 2090129c36c7e582943b7d300968d19b46160d84 causing:

    [...]/gdb/gnu-nat.c: In function 'void gnu_ptrace_me()':
    [...]/gdb/gnu-nat.c:2133:5: error: 'trace_start_error_with_name' was not declared in this scope
         trace_start_error_with_name ("ptrace");
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    [...]/gdb/gnu-nat.c:2133:5: note: suggested alternative: 'throw_perror_with_name'
         trace_start_error_with_name ("ptrace");
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
         throw_perror_with_name
    [...]/gdb/gnu-nat.c: In function 'void gnu_create_inferior(target_ops*, const char*, const string&, char**, int)':
    [...]/gdb/gnu-nat.c:2147:9: error: 'fork_inferior' was not declared in this scope
       pid = fork_inferior (exec_file, allargs, env, gnu_ptrace_me,
             ^~~~~~~~~~~~~
    [...]/gdb/gnu-nat.c:2147:9: note: suggested alternative: 'exit_inferior'
       pid = fork_inferior (exec_file, allargs, env, gnu_ptrace_me,
             ^~~~~~~~~~~~~
             exit_inferior
    [...]/gdb/gnu-nat.c:2174:30: error: 'START_INFERIOR_TRAPS_EXPECTED' was not declared in this scope
       gdb_startup_inferior (pid, START_INFERIOR_TRAPS_EXPECTED);
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    /usr/bin/ld: gnu-nat.o: in function `gnu_ptrace_me()':
    [...]/gdb/gnu-nat.c:2134: undefined reference to `trace_start_error_with_name(char const*)'
    /usr/bin/ld: gnu-nat.o: in function `gnu_create_inferior(target_ops*, char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char**, int)':
    [...]/gdb/gnu-nat.c:2148: undefined reference to `fork_inferior(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char**, void (*)(), void (*)(int), void (*)(), char const*, void (*)(char const*, char* const*, char* const*))'
    /usr/bin/ld: fork-child.o: in function `gdb_startup_inferior(int, int)':
    [...]/gdb/fork-child.c:136: undefined reference to `startup_inferior(int, int, target_waitstatus*, ptid_t*)'
    collect2: error: ld returned 1 exit status

	gdb/
	* configure.nat [gdb_host == i386gnu] (NATDEPFILES): Add
	'nat/fork-inferior.o'.
	* gnu-nat.c: #include "nat/fork-inferior.h".