unix: test uv_barrier_t size in right block (libuv#4311)

The local uv_barrier_t implementation is used: for AIX, OpenBSD and any system
not defining PTHREAD_BARRIER_SERIAL_THREAD.

As the uv_barrier_t struct was modified and it isn't a pointer anymore, the
check for uv_barrier_t size isn't right anymore for systems not using local
uv_barrier_t implementation. so move the check inside the block implementing
uv_barrier_t using pthread_barrier_t.

Tested on OpenBSD, where the build failed due to the STATIC_ASSERT().

src/thread-common.c

@@ -27,10 +27,6 @@
 #include <pthread.h>
 #endif
 
-#if defined(PTHREAD_BARRIER_SERIAL_THREAD)
-STATIC_ASSERT(sizeof(uv_barrier_t) == sizeof(pthread_barrier_t));
-#endif
-
 /* Note: guard clauses should match uv_barrier_t's in include/uv/unix.h. */
 #if defined(_AIX) || \
     defined(__OpenBSD__) || \
@@ -113,6 +109,8 @@ void uv_barrier_destroy(uv_barrier_t* barrier) {
 
 #else
 
+STATIC_ASSERT(sizeof(uv_barrier_t) == sizeof(pthread_barrier_t));
+
 int uv_barrier_init(uv_barrier_t* barrier, unsigned int count) {
   return UV__ERR(pthread_barrier_init(barrier, NULL, count));
 }