.gitignore: ignore editor backup files

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/.gitignore b/.gitignore
index d2cff7c..c0d5183 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 cscope.*
 *.swp
 *.patch
+*~
 .project
 .cproject