Include pthread.h when using GOOGLE_PROTOBUF_NO_THREADLOCAL.

When GOOGLE_PROTOBUF_NO_THREADLOCAL is defined, classes that depend on
pthread functions are included (such as ThreadLocalStorage).

src/google/protobuf/stubs/platform_macros.h

@@ -115,6 +115,8 @@ 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_