winpthreads: don't call SetThreadContext in winstore builds

The call is forbidden.

Hopefully the deferred Canceling mechanism below will work and this is not
needed.

Signed-off-by: Liu Hao <lh_mouse@126.com>
diff --git a/mingw-w64-libraries/winpthreads/src/thread.c b/mingw-w64-libraries/winpthreads/src/thread.c
index 142f65e..8afa90f 100644
--- a/mingw-w64-libraries/winpthreads/src/thread.c
+++ b/mingw-w64-libraries/winpthreads/src/thread.c
@@ -1256,7 +1256,9 @@
 #else
 #error Unsupported architecture
 #endif
+#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
 	  SetThreadContext (tv->h, &ctxt);
+#endif
 
 	  /* Also try deferred Cancelling */
 	  tv->cancelled = 1;