Exclude based on __APPLE__

Summary: This was showing up on a Linux compile to my sadness.

Reviewed By: francis-ma

Differential Revision: D2876010

fb-gh-sync-id: f638d593712c0aeb50177d96aaacb26575820359

folly/Portability.h

@@ -231,7 +231,7 @@ namespace std { typedef ::max_align_t max_align_t; }
 # error cannot define platform specific thread local storage
 #endif
 
-#if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE
+#if defined(__APPLE__) && (TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE)
 #undef FOLLY_TLS
 #endif