spec: add missing build dependency

Otherwise:

/usr/bin/perl Makefile.PL PREFIX='/opt/git' INSTALL_BASE=''
Can't locate ExtUtils/MakeMaker.pm in @inc (@inc contains: ...) at Makefile.PL line 1.
BEGIN failed--compilation aborted at Makefile.PL line 1.
make[1]: *** [perl.mak] Error 2
make: *** [perl/perl.mak] Error 2

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.spec.in

@@ -101,6 +101,7 @@ Group:          Development/Libraries
 Requires:       git = %{version}-%{release}
 Requires:       perl(:MODULE_COMPAT_%(eval "`%{__perl} -V:version`"; echo $version))
 BuildRequires:  perl(Error)
+BuildRequires:  perl(ExtUtils::MakeMaker)
 
 %description -n perl-Git
 Perl interface to Git