gitignore: Don't ignore 'tags' directory

W=1 builds reported warnings regarding files being ignored:
   tools/testing/selftests/arm64/tags/.gitignore: warning: ignored by one of the .gitignore files
   tools/testing/selftests/arm64/tags/Makefile: warning: ignored by one of the .gitignore files
   tools/testing/selftests/arm64/tags/tags_test.c: warning: ignored by one of the .gitignore files

Adjusting the .gitignore entries will prevent these warnings and
ensure a smoother script execution.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

.gitignore

@@ -128,6 +128,7 @@ series
 
 # ctags files
 tags
+!tags/
 TAGS
 
 # cscope files