gitignore: add m4 directory

New autotools directory: m4, add to .gitignore.

Change-Id: Idc6d0f7c53927c62602a5caa16010941d5a75b33
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/.gitignore b/.gitignore
index 91e40cd..dde04e4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -26,6 +26,7 @@
 /install-sh
 /missing
 /stamp-h1
+/m4
 
 # https://www.gnu.org/software/libtool/