tests/hnb-test-ranap: Fix incorrect pointer type cast

osmo_hexdump expect it to be a const unsigned char*, and gcc warns about
incorrect signedness.

Change-Id: I7e0622f502a4442d700340b0f1c2f8cd01e90ecd

src/tests/hnb-test-ranap.c

@@ -5,7 +5,7 @@
 
 static const char *printstr(OCTET_STRING_t *s)
 {
-	return osmo_hexdump((char*)s->buf, s->size);
+	return osmo_hexdump((const unsigned char*)s->buf, s->size);
 }
 
 #define PP(octet_string_t) \