Makefile: silence perl/PM.stamp recipe

Every time we run "make", we update perl/PM.stamp, which
contains a list of all of the perl module files (if it's
updated, we need to rebuild perl/perl.mak, since the
Makefile will not otherwise know about the new files).

This means that every time "make" is run, we see:

      GEN perl/PM.stamp

in the output, even though it is not likely to have changed.
Let's make this recipe completely silent, as we do for other
auto-generated dependency files (e.g., GIT-CFLAGS).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1685,7 +1685,7 @@ $(SCRIPT_PERL_GEN): perl/perl.mak
 perl/perl.mak: perl/PM.stamp
 
 perl/PM.stamp: FORCE
-	$(QUIET_GEN)$(FIND) perl -type f -name '*.pm' | sort >$@+ && \
+	@$(FIND) perl -type f -name '*.pm' | sort >$@+ && \
 	{ cmp $@+ $@ >/dev/null 2>/dev/null || mv $@+ $@; } && \
 	$(RM) $@+