pacakge/tvheadend: fix failures due to -Werror

(Partially) apply upstream patch 1334869bf0ea668e58e1c72daa3c09c1a95b5d81
to properly protect code-paths depending on execinfo.h (for backtrace(3)).

Fixes:
    http://autobuild.buildroot.net/results/fe25485bde8894daff5bdf806eb4bb2fa776dc55/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>