package/jack2: fix build WRT backtrace support

This change adds a patch checking for the presence of execinfo.h header
and enabling the backtrace support depending on the check result.

Fixes:
  http://autobuild.buildroot.org/results/415/415e2100dc59d35e58646c07f7cdccabecdda966/
  http://autobuild.buildroot.org/results/43c/43ca1b103434ae582fdf93cb5912b311960f303b/
  http://autobuild.buildroot.org/results/391/391e71a988250ea66ec4dbee6f60fdce9eaf2766/
  ...

Adapted from the PR:
  https://github.com/jackaudio/jack2/pull/206



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>