funcs/func_holdintercept: Actually add the HOLD_INTERCEPT function

When ab803ec3 was committed, it accidentally forgot to actually *add* the
HOLD_INTERCEPT function. This highlights two interesting points:
* Gerrit forces you to put the patch as it is going to into the repo up for
  review, which Review Board did not. Yay Gerrit.
* No one apparently bothered to use this feature, or else they don't know about
  it. I'm going to go with the latter explanation.

ASTERISK-24922

Change-Id: Ida38278f259dd07c334a36f9b7d5475b5db72396