Forgot to fix the guard for String.cpp

Summary:9ab69bc7 removed `__APPLE__` around the strndup definition
because it is defined on modern OSX.  But there was another reference
in String.cpp. Fixing that.

Reviewed By: yfeldblum

Differential Revision: D3186360

fb-gh-sync-id: 709c3b93cd22945e2237412637929df1979526c1
fbshipit-source-id: 709c3b93cd22945e2237412637929df1979526c1

folly/portability/String.cpp

@@ -27,7 +27,7 @@ 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) {
   auto neededLen = strlen(a);
   if (neededLen > len) {