MINOR: sample: converter: Add mjson library.

This library is required for the subsequent patch which adds
the JSON query possibility.

It is necessary to change the include statement in "src/mjson.c"
because the imported includes in haproxy are in "include/import"

orig: #include "mjson.h"
new:  #include <import/mjson.h>
diff --git a/Makefile b/Makefile
index 9b22fe4..5592488 100644
--- a/Makefile
+++ b/Makefile
@@ -883,7 +883,8 @@
         src/ebistree.o src/auth.o src/wdt.o src/http_acl.o                     \
         src/hpack-enc.o src/hpack-huff.o src/ebtree.o src/base64.o             \
         src/hash.o src/dgram.o src/version.o src/fix.o src/mqtt.o src/dns.o    \
-        src/server_state.o src/proto_uxdg.o src/init.o src/cfgdiag.o
+        src/server_state.o src/proto_uxdg.o src/init.o src/cfgdiag.o           \
+        src/mjson.o
 
 ifneq ($(TRACE),)
 OBJS += src/calltrace.o