.gitignore: ignore files ending with ~

I'm lately getting this kind of files in git after building:
config.guess~
config.sub~
configure~
install-sh~

Let's ignore them.

Change-Id: I976e3a33f638f4cd19650b9c799e61713d73bf8a
diff --git a/.gitignore b/.gitignore
index 01f4f47..c8b1d06 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,6 +20,7 @@
 missing
 *libtool
 ltmain.sh
+*~
 
 core
 core.*