git 1.7.7: remove perl.mak before compile

git 1.7.7: remove perl.mak before compile

The git may fail to rebuild when perl's Config.pm or config.h changes,
this is because Makefile detects that perl/perl.mak is out of date.
Remove perl.mak to let Makefile regenerate it would fix the error.
Both git and git-native have this problem.

To reproduce the error: (On x86_64 host)

$ bitbake git-native
$ touch tmp/sysroots/x86_64-linux/usr/lib/perl-native/perl/5.14.2/Config.pm
$ bitbake git-native -ccompile -f

[YOCTO #2156]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>