Use #warning rather than #warn

Summary: Because `#warn` isn't actually a thing.

Reviewed By: ot

Differential Revision: D5356878

fbshipit-source-id: 57751ca57e7f6aaf9f4d8be70958b9cacc4f7335

folly/memory/UninitializedMemoryHacks.h

@@ -244,7 +244,7 @@ inline void unsafeStringSetLargerSize(std::string& s, std::size_t n) {
 }
 
 #else
-#warn "No implementation for resizeWithoutInitialization of std::string"
+#warning "No implementation for resizeWithoutInitialization of std::string"
 #endif
 
 // This machinery bridges template expansion and macro expansion