Turn on -Wmissing-prototypes

Summary: Compiling for iOS has by default turned on -Wmissing-prototypes, which causes rocksdb to fail compiling. This diff turns on -Wmissing-prototypes in our compile options and cleans up all functions with missing prototypes.

Test Plan: compiles

Reviewers: dhruba, haobo, ljin, sdong

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17649

util/string_util.h

@@ -3,9 +3,13 @@
 //  LICENSE file in the root directory of this source tree. An additional grant
 //  of patent rights can be found in the PATENTS file in the same directory.
 //
+#include <sstream>
+#include <string>
+#include <vector>
+
 #pragma once
 namespace rocksdb {
 
 extern std::vector<std::string> stringSplit(std::string arg, char delim);
 
-}
+}  // namespace rocksdb