libobs: Add windowless context #error

Intentionally breaks compilation when trying to compile the specific
merged commits within the windowless-context branch.  This is meant to
be used in conjunction with a merge commit so that bisecting will never
see any non-compiling commits.

libobs/obs.h

@@ -16,6 +16,7 @@
 ******************************************************************************/
 
 #pragma once
+#error "libobs is not compilable in this commit (windowless context)"
 
 #include "util/c99defs.h"
 #include "util/bmem.h"