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

third-party/pjproject/Makefile.rules

@@ -37,7 +37,7 @@ PJPROJECT_CONFIG_OPTS = $(PJPROJECT_CONFIGURE_OPTS) --prefix=/opt/pjproject \
 	--disable-ipp \
 	--disable-libwebrtc \
 	--without-external-pa \
-	--with-external-srtp
+	--without-external-srtp
 
 ifneq ($(AST_DEVMODE),yes)
     PJPROJECT_CONFIG_OPTS += --disable-resample --disable-g711-codec