Fix perl/ build.

An earlier commit f848718a broke the build in perl/ directory by
allowing the Makefile.PL to overwrite the now-tracked Makefile.
Fix this by forcing Makefile.PL to produce its output in
perl.mak as the broken commit originally intended.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/perl/Makefile.PL b/perl/Makefile.PL
index de73235..4168775 100644
--- a/perl/Makefile.PL
+++ b/perl/Makefile.PL
@@ -24,5 +24,6 @@
 	NAME            => 'Git',
 	VERSION_FROM    => 'Git.pm',
 	PM		=> \%pm,
+	MAKEFILE	=> 'perl.mak',
 	%extra
 );