need to include vmb.h unconditionally

it is the place which defines
VMB_NEEDS_PTHREAD_WORKAROUND_THIS_IS_BAD_FOR_PERFORMANCE

lib/libvarnish/vmb.c

@@ -28,13 +28,14 @@
 
 #include "config.h"
 
+#include "vmb.h"
+
 #ifdef VMB_NEEDS_PTHREAD_WORKAROUND_THIS_IS_BAD_FOR_PERFORMANCE
 
 #include <pthread.h>
 #include <unistd.h>
 
 #include "vas.h"
-#include "vmb.h"
 
 static pthread_mutex_t	mb_mtx;
 static pthread_once_t	mb_mtx_once = PTHREAD_ONCE_INIT;