net/asterisk16: Fix configure issue when PJSIP option is disabled

pkgconfig is now used by asterisk to detect most required dependencies,
so add it to the global USES.

At the same time there is a bug in the menuselect configure script
that shows up when the PJSIP option is disabled. It tries to run
sed through a $SED variable before populating it.

As a workaround for now I just replace the $SED variable with the
bare sed command.

PR: 263980
Fixes: e8e6fba878cf12