CONTRIB: hpack: add an hpack decoder

This decoder takes a series of hex codes on stdin using one line
per HEADERS frame and shows the decoded headers.
diff --git a/contrib/hpack/Makefile b/contrib/hpack/Makefile
index 1ecc4e4..128a6de 100644
--- a/contrib/hpack/Makefile
+++ b/contrib/hpack/Makefile
@@ -1,5 +1,5 @@
-CFLAGS = -O2 -W -Wall -Wextra -g
-OBJS = gen-rht
+CFLAGS = -O2 -Wall -g -I../../include -I../../ebtree -fwrapv -fno-strict-aliasing
+OBJS = gen-rht decode
 
 all: $(OBJS)