Escape '.' by adding a '\' to avoid matching any char

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/4912

Differential Revision: D13789449

Pulled By: riversand963

fbshipit-source-id: 0639dae82049b7ac977c8f81851f1c9fdc346705

Makefile

@@ -198,7 +198,7 @@ include make_config.mk
 CLEAN_FILES += make_config.mk
 
 missing_make_config_paths := $(shell				\
-	grep "./\S*\|/\S*" -o $(CURDIR)/make_config.mk | 	\
+	grep "\./\S*\|/\S*" -o $(CURDIR)/make_config.mk | 	\
 	while read path;					\
 		do [ -e $$path ] || echo $$path; 		\
 	done | sort | uniq)