Fix macOS build, missing override

Fix macOS build, missing override

Fixes:

In file included from ../../src/gdb/darwin-nat.c:63:0:
../../src/gdb/darwin-nat.h:28:8: error: 'virtual void darwin_nat_target::create_inferior(const char*, const string&, char**, int)' can be marked override [-Werror=suggest-override]
   void create_inferior (const char *exec_file,
        ^

gdb/ChangeLog:
2018-05-24  Pedro Alves  <palves@redhat.com>

	* darwin-nat.h (darwin_nat_target::create_inferior): Mark with
	override.