ignore new scratch directory for metadata

This directory is created by ExtUtils::MakeMaker starting with version
7.05_05, via this commit:
b6a9eba5b0

It was added to the default MANIFEST.SKIP in ExtUtils-Manifest 1.70, here:
e0f44577f8

Perl.gitignore

@@ -17,3 +17,4 @@ nytprof.out
 /pm_to_blib
 *.o
 *.bs
+/_eumm/