buildroot

libpjsip: really disable gsm

--disable-gsm does not exist, the option is called --disable-gsm-codec.

Fixes:
  http://autobuild.buildroot.org/results/f8c/f8c41f35ab56fac86f2877b09a60924256f57028/

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>