$(libexecdir) isn't used anywhere else in the project, while
$(gitexecdir) is the standard in the other appropriate makefiles. Hence,
replace the former with the latter.
Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: James Denholm <nod.helm@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>