app_voicemail: Add workaround for a gcc 10 issue with -Wrestrict

app_voicemail: Add workaround for a gcc 10 issue with -Wrestrict

The gcc 10 -Wrestrict option was causing "overlap" errors when
snprintf was copying one char[256] structure member to another
char[256] member in the same structure.

Using ast_alloca instead of declaring the structure inline
solves the issue.

Here's a link to the "enhancement":
https://gcc.gnu.org/legacy-ml/gcc-patches/2019-10/msg00570.html

We may follow up with a gcc bug report.

Change-Id: Ie0099adcb0a9727bd9aa99e024dd912a67eaf534