build: Ignore new common/mldrivers in maintainer-check-extra-dist

This directory is not distributed in the tarball.

Fixes: commit 1a6485001c

Makefile.am

@@ -443,6 +443,7 @@ maintainer-check-extra-dist:
 	  cd common; git ls-files | sed 's,^,common/,' ) | \
 	  grep -v '^common$$' | \
 	  grep -v '^common/mlcustomize/' | \
+	  grep -v '^common/mldrivers/' | \
 	  grep -v '^common/mlgettext/' | \
 	  grep -v '^common/mlprogress/' | \
 	  grep -v '^common/mltools/' | \