tools/vchan: Fix -Wsingle-bit-bitfield-constant-conversion

Gitlab reports:

  node.c:158:17: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]

        ctrl->blocking = 1;
                       ^ ~
  1 error generated.
  make[4]: *** [/builds/xen-project/people/andyhhp/xen/tools/vchan/../../tools/Rules.mk:188: node.o] Error 1

In Xen 4.18, this was fixed with c/s 99ab02f63e ("tools: convert bitfields
to unsigned type") but this is an ABI change which can't be backported.

Swich 1 for -1 to provide a minimally invasive way to fix the build.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

tools/vchan/node.c

@@ -155,7 +155,7 @@ int main(int argc, char **argv)
 		perror("libxenvchan_*_init");
 		exit(1);
 	}
-	ctrl->blocking = 1;
+	ctrl->blocking = -1;
 
 	srand(seed);
 	fprintf(stderr, "seed=%d\n", seed);