Dirty quick fix for implicit function check, VKD3D_CHECK_FUNC is
unsuitable to handle it.

Update: https://gitlab.winehq.org/wine/vkd3d/-/merge_requests/528
--- a/configure
+++ b/configure
@@ -18656,2 +18656,3 @@
 /* end confdefs.h.  */
+#include <unistd.h>
 int main(void) { return gettid(); }