Generate tags for *.c files

Summary:
db/c_test.c uses the functions in db/c.cc. If we have tags generated
for one but not the other, it's easy to make mistakes like updating a function
signature and missing a call site.

Test Plan:
  $ make tags

in vim:

  :cscope find s rocksdb_options_set_compression_options
  ...
  3    325  db/c_test.c <<main>>
            rocksdb_options_set_compression_options(options, -14, -1, 0);

Reviewers: sdong, yhchiang, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D53685

Makefile

@@ -700,7 +700,7 @@ clean:
 
 tags:
 	ctags * -R
-	cscope -b `find . -name '*.cc'` `find . -name '*.h'`
+	cscope -b `find . -name '*.cc'` `find . -name '*.h'` `find . -name '*.c'`
 
 format:
 	build_tools/format-diff.sh