`strndup` is defined on modern OSX

Summary: `strndup` is defined on modern versions of OSX

Reviewed By: Orvid, grp

Differential Revision: D3122635

fb-gh-sync-id: 678792765addf2fb226e835d3b7a67e155ed6dc5
fbshipit-source-id: 678792765addf2fb226e835d3b7a67e155ed6dc5

folly/portability/String.h

@@ -25,7 +25,7 @@
 extern "C" void* memrchr(const void* s, int c, size_t n);
 #endif
 
-#if defined(_WIN32) || defined(__APPLE__) || defined(__FreeBSD__)
+#if defined(_WIN32) || defined(__FreeBSD__)
 extern "C" char* strndup(const char* a, size_t len);
 #endif