Dpkg::BuildFlags: Use =head2 instead of =head

This fixes a build failure with Test::Pod installed.

Regression introduced in commit 78ffee002f47daed249e62778587383f2cd2a21a.

scripts/Dpkg/BuildFlags.pm

@@ -390,7 +390,7 @@ the prepend operation everywhere.
 New method: $bf->load_maintainer_config() that update the build flags
 based on the package maintainer directives.
 
-=head Version 1.02
+=head2 Version 1.02
 
 New methods: $bf->get_features(), $bf->has_features(), $bf->set_feature().