runtime/testdata/testprogcgo: fix Windows C compiler warning

Noticed and fix by Alex Brainman.

Tested in https://golang.org/cl/23005 (which makes all compiler
warnings fatal during development)

Fixes #15623

Change-Id: Ic19999fce8bb8640d963965cc328574efadd7855
Reviewed-on: https://go-review.googlesource.com/23010Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/runtime/testdata/testprogcgo/threadpanic_windows.c

@@ -8,7 +8,7 @@
 
 void gopanic(void);
 
-static unsigned int
+static unsigned int __attribute__((__stdcall__))
 die(void* x)
 {
 	gopanic();