apply.h: include missing header

apply.h: include missing header

When running `make hdr-check`, we got the following error messages:

	apply.h:146:22: error: use of undeclared identifier 'GIT_MAX_HEXSZ'
		char old_oid_prefix[GIT_MAX_HEXSZ + 1];
				    ^
	apply.h:147:22: error: use of undeclared identifier 'GIT_MAX_HEXSZ'
		char new_oid_prefix[GIT_MAX_HEXSZ + 1];
				    ^
	apply.h:151:33: error: array has incomplete element type 'struct object_id'
		struct object_id threeway_stage[3];
					       ^
	./strbuf.h:79:8: note: forward declaration of 'struct object_id'
	struct object_id;
	       ^
	3 errors generated.
	make: *** [apply.hco] Error 1

Include the missing "hash.h" header to fix these errors.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>