Fix compilation warning about hamlib_check_rig_caps

include/hamlib/rig.h

@@ -2006,7 +2006,7 @@ struct rig_caps {
                                size_t frame_length,
                                const unsigned char *frame);
 // this will be used to check rigcaps structure is compatible with client
-    const char *hamlib_check_rig_caps;   // a constant value we can check for hamlib integrity
+    char *hamlib_check_rig_caps;   // a constant value we can check for hamlib integrity
     int (*get_conf2)(RIG *rig, token_t token, char *val, int val_len);
 };
 //! @endcond