add 'm4' subdirectory to .gitignore

Change-Id: I0207549ed00ad9c95ed64978046bb53ffe2634fe
diff --git a/.gitignore b/.gitignore
index 041cf24..7289d72 100644
--- a/.gitignore
+++ b/.gitignore
@@ -18,6 +18,7 @@
 libgtpnl.pc
 libtool
 build-aux
+m4
 
 # Debian package build temporary files
 build-stamp