aix: Add fork-inferior.o to NATDEPFILES

aix: Add fork-inferior.o to NATDEPFILES

Trying to build on AIX gives:

ld: 0711-317 ERROR: Undefined symbol: .trace_start_error_with_name(char const*)
ld: 0711-317 ERROR: Undefined symbol: .fork_inferior(char const*, std::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*))
ld: 0711-317 ERROR: Undefined symbol: .startup_inferior(int, int, target_waitstatus*, ptid_t*)

Including fork-inferior.o in the build should help.  I also factored out
the AIX bits that are not architecture-specific to be consistent with the other
OSes.

gdb/ChangeLog:

	* configure.nat: Factor out AIX bits that are not
	architecture-specific.  Add fork-inferior.o.