bundled_pjproject: Fix srtp detection

Reverted recent change that set '--with-external-srtp' instead
of '--without-external-srtp'.  Since Asterisk handles all SRTP,
we don't need it enabled in pjproject at all.

ASTERISK-29867

Change-Id: I2ce1bdd30abd21c062eac8f8fefe9b898787b801