libpjsip: depend on libsrtp

libpjsip: depend on libsrtp

libpjsip needs libsrtp to build, but it bundles a libsrtp version in
case one is not found during the configure step. The Buildroot policy
in such cases is to force using the external package, so forcibly
depend on libsrtp.

Adding --with-external-srtp also fixes libpjsip not correctly the
libsrtp installed in staging, which shows up with the symptom:

  .../libpjmedia.so: undefined reference to `srtp_deinit'
  collect2: error: ld returned 1 exit status

Fixes:
  http://autobuild.buildroot.org/results/305/305fdc8442cd2e8f51b90485be0dca83ffa36603/
  http://autobuild.buildroot.org/results/a2f/a2f407c1361ac5c24af122445e84645e9aee309d/
  ...and many other similar autobuild failures.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>