Move pthread include.

Based on pull request feedback. See commit 60c52222 for why the pthread
header is necessary.

src/google/protobuf/stubs/mutex.h

@@ -30,6 +30,10 @@
 #ifndef GOOGLE_PROTOBUF_STUBS_MUTEX_H_
 #define GOOGLE_PROTOBUF_STUBS_MUTEX_H_
 
+#ifdef GOOGLE_PROTOBUF_NO_THREADLOCAL
+#include <pthread.h>
+#endif
+
 #include <google/protobuf/stubs/macros.h>
 
 // ===================================================================

src/google/protobuf/stubs/platform_macros.h

@@ -115,8 +115,6 @@ GOOGLE_PROTOBUF_PLATFORM_ERROR
 // TLS support on android.
 // iOS also does not support the __thread keyword.
 #define GOOGLE_PROTOBUF_NO_THREADLOCAL
-
-#include <pthread.h>
 #endif
 
 #endif  // GOOGLE_PROTOBUF_PLATFORM_MACROS_H_