.gitignore: move tags and TAGS close to other tag files

For consistency, move tags and TAGS close to the cscope and GNU Global
patterns.

I removed the '/' prefix in case somebody wants to manually create tag
files in sub-directories.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

.gitignore

@@ -54,8 +54,6 @@ modules.order
 #
 # Top-level generic files
 #
-/tags
-/TAGS
 /linux
 /modules-only.symvers
 /vmlinux
@@ -114,6 +112,10 @@ patches-*
 patches
 series
 
+# ctags files
+tags
+TAGS
+
 # cscope files
 cscope.*
 ncscope.*