BUG/MINOR: stdarg.h inclusion

Needed for the memvprintf part, the va_list type.
Spotted during OpenBSD build.
diff --git a/include/common/standard.h b/include/common/standard.h
index 59809b4..a06fb84 100644
--- a/include/common/standard.h
+++ b/include/common/standard.h
@@ -26,6 +26,7 @@
 #include <string.h>
 #include <stdio.h>
 #include <time.h>
+#include <stdarg.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/socket.h>