compiler.h needs to define __must_check if not set

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/compiler.h b/compiler.h
index 8a3fe63..72e8419 100644
--- a/compiler.h
+++ b/compiler.h
@@ -9,4 +9,8 @@
 #error Compiler too old, need gcc at least gcc 3.x
 #endif
 
+#ifndef __must_check
+#define __must_check
+#endif
+
 #endif