Add overrides, fix FreeBSD build

Fix this:

  CXX    fbsd-nat.o
In file included from fbsd-nat.c:44:
./fbsd-nat.h:40:7: error: 'find_memory_regions' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  int find_memory_regions (find_memory_region_ftype func, void *data);
      ^
./target.h:702:17: note: overridden virtual function is here
    virtual int find_memory_regions (find_memory_region_ftype func, void *data)
                ^
In file included from fbsd-nat.c:44:
./fbsd-nat.h:42:8: error: 'info_proc' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  bool info_proc (const char *, enum info_proc_what);
       ^
./target.h:950:18: note: overridden virtual function is here
    virtual bool info_proc (const char *, enum info_proc_what);
                 ^

gdb/ChangeLog:

	* fbsd-nat.h (class fbsd_nat_target) <find_memory_regions>: Add
	override.
	<info_proc>: Likewise.