vxp: Missing include for in64_t

Spotted by our Alpine Linux job on CircleCI.

lib/libvarnishapi/vxp.c

@@ -33,6 +33,7 @@
 #include "config.h"
 
 #include <ctype.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h> /* for MUSL */