.gitignore: add leading and trailing slashes to generated directories

Clarify these directory paths are relative to the top of the source
tree.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

.gitignore

@@ -90,10 +90,10 @@ modules.builtin
 #
 # Generated include files
 #
-include/config
-include/generated
-include/ksym
-arch/*/include/generated
+/include/config/
+/include/generated/
+/include/ksym/
+/arch/*/include/generated/
 
 # stgit generated dirs
 patches-*