gitignore: Add m4 scripts from m4 subdir

Change-Id: I8da2a55e84bcc24cc5af00dc089630a18105c625
diff --git a/.gitignore b/.gitignore
index a9e6b84..7633c73 100644
--- a/.gitignore
+++ b/.gitignore
@@ -38,6 +38,7 @@
 stamp-h1
 libtool
 ltmain.sh
+m4/*.m4
 
 # git-version-gen magic
 .tarball-version