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

db/plain_table_db_test.cc

@@ -429,9 +429,11 @@ TEST(PlainTableDBTest, Iterator) {
   }
 }
 
+namespace {
 std::string MakeLongKey(size_t length, char c) {
   return std::string(length, c);
 }
+}  // namespace
 
 TEST(PlainTableDBTest, IteratorLargeKeys) {
   Options options = CurrentOptions();