enum.c: Make ast_get_txt() actually do something.

The ast_get_txt() API function (and by extension, the TXTCIDNAME
dialplan function) were broken in
65b83815 such that we would never
actually make a DNS TXT query as described.

This patch restores the documented behavior.

ASTERISK-19460 #close
Reported by: George Joseph

Change-Id: I1b19aea711488cb1ecd63843cddce05010e39376