BUILD/DEBUG: hpack: use unsigned int in printf format in debug code

In issue #1184 cppcheck found that the debug code incorrectly uses %d
to print an unsigned value.
diff --git a/src/hpack-dec.c b/src/hpack-dec.c
index 4fa9bfd..6e4a8b7 100644
--- a/src/hpack-dec.c
+++ b/src/hpack-dec.c
@@ -166,7 +166,7 @@
 		if (*raw >= 0x80) {
 			/* indexed header field */
 			if (*raw == 0x80) {
-				hpack_debug_printf("unhandled code 0x%02x (raw=%p, len=%d)\n", *raw, raw, len);
+				hpack_debug_printf("unhandled code 0x%02x (raw=%p, len=%u)\n", *raw, raw, len);
 				ret = -HPACK_ERR_UNKNOWN_OPCODE;
 				goto leave;
 			}