MINOR: qpack: implement standalone decoder tool

Implement a standalone binary to be able to easily a hex-string QPACK
stream. The binary must be compiled via the Makefile. Hex-strings are
specified on stdin.
diff --git a/Makefile b/Makefile
index c852d9d..b248acc 100644
--- a/Makefile
+++ b/Makefile
@@ -1018,6 +1018,9 @@
 dev/poll/poll:
 	$(Q)$(MAKE) -C dev/poll poll CC='$(cmd_CC)' OPTIMIZE='$(COPTS)'
 
+dev/qpack/decode: dev/qpack/decode.o
+	$(cmd_LD) $(LDFLAGS) -o $@ $^ $(LDOPTS)
+
 dev/tcploop/tcploop:
 	$(Q)$(MAKE) -C dev/tcploop tcploop CC='$(cmd_CC)' OPTIMIZE='$(COPTS)'
 
@@ -1088,6 +1091,7 @@
 	$(Q)rm -f dev/*/*.[oas]
 	$(Q)rm -f dev/flags/flags dev/poll/poll dev/tcploop/tcploop
 	$(Q)rm -f dev/hpack/decode dev/hpack/gen-enc dev/hpack/gen-rht
+	$(Q)rm -f dev/qpack/decode
 
 tags:
 	$(Q)find src include \( -name '*.c' -o -name '*.h' \) -print0 | \