Compatibility with current varnish-cache

vrt_null_blob was moved into varnishd and thus is only available
when a vmod is dlopen'ed from varnishd.

Provide an own implementation and make sure it is exported.

README.rst

@@ -2,7 +2,7 @@
 vmod-tus
 ========
 
-This version is for Varnish-Cache 6.5.0 and later
+This version is for Varnish-Cache 7 and later
 
 DESCRIPTION
 ===========

src/Makefile.am

@@ -8,6 +8,7 @@ bin_PROGRAMS = vmod_get_h
 vmod_get_h_SOURCES = \
 	vmod_get_h.c
 
+vmod_get_h_LDFLAGS = -rdynamic
 vmod_get_h_LDADD = -ldl
 
 TESTS = \

src/vmod_get_h.c

@@ -36,6 +36,12 @@
 #include "vdef.h"
 #include "vrt.h"
 
+const struct vrt_blob *vrt_null_blob = &(struct vrt_blob){
+	.type = VRT_NULL_BLOB_TYPE,
+	.len = 0,
+	.blob = "\0"
+};
+
 int
 main(int argc, char **argv) {
 	void *dlhdl;