build warnings

[seven1240]

I already fixed some build warnings in #92

the following warnings seems from the new identity change which I'm not sure.

sip_basic.c:2885:15: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
          uri = pp + 1;
              ^ ~~~~~~
sip_basic.c:2905:40: warning: passing 'msg_param_t **' (aka 'const char ***') to parameter of type 'const msg_param_t **' (aka 'const char *const **') discards qualifiers in nested pointer types [-Wincompatible-pointer-types-discards-qualifiers]
          if (msg_params_d(home, &ext, &params) >= 0) {
                                       ^~~~~~~
./../msg/sofia-sip/msg_parser.h:280:27: note: passing argument to parameter 'return_params' here
                                  msg_param_t const **return_params);