msgb: Add non-inlined helper functions to ease with language bindings

It is easier from languages that dlopen libosmocore to have a function
to access this data than to poke at the bytes of the struct directly. Add
copyright for this method and the reset method I did earlier this year.
diff --git a/include/osmocore/msgb.h b/include/osmocore/msgb.h
index 354c5d8..395c7c2 100644
--- a/include/osmocore/msgb.h
+++ b/include/osmocore/msgb.h
@@ -187,4 +187,9 @@
 	return msg;
 }
 
+/* non inline functions to ease binding */
+uint8_t *msgb_data(const struct msgb *msg);
+uint16_t msgb_length(const struct msgb *msg);
+
+
 #endif /* _MSGB_H */