osmo_msgbdump_{l2,l3}(): Proper typecast

This avoids compiler warnings like

/tmp/work/sysmobts_v2-poky-linux-gnueabi/osmo-pcu/0.4+gitAUTOINC+4c112dc5a6-r1.18/recipe-sysroot/usr/include/osmocom/core/msgb.h: In function 'const char* msgb_hexdump_l2(const msgb*)':
error: invalid conversion from 'void*' to 'const unsigned char*' [-fpermissive]
    return osmo_hexdump(msgb_l2(msg), msgb_l2len(msg));

which we've been getting in osmo-pcu builds on some platforms.

Change-Id: I0ec652a1a569ec1507d8411cf1ef87afabcca799
diff --git a/include/osmocom/core/msgb.h b/include/osmocom/core/msgb.h
index 9f4c444..374ddbe 100644
--- a/include/osmocom/core/msgb.h
+++ b/include/osmocom/core/msgb.h
@@ -513,14 +513,14 @@
 {
 	if (!msgb_l2(msg) || !(msgb_l2len(msg)))
 		return "[]";
-	return osmo_hexdump(msgb_l2(msg), msgb_l2len(msg));
+	return osmo_hexdump((const unsigned char *) msgb_l2(msg), msgb_l2len(msg));
 }
 
 static inline const char *msgb_hexdump_l3(const struct msgb *msg)
 {
 	if (!msgb_l3(msg) || !(msgb_l3len(msg)))
 		return "[]";
-	return osmo_hexdump(msgb_l3(msg), msgb_l3len(msg));
+	return osmo_hexdump((const unsigned char*) msgb_l3(msg), msgb_l3len(msg));
 }
 
 /*! @} */