SystemInformation: Include backtrace-related headers on FreeBSD

This was probably broken for a long while, but the problem was not apparent
because the check for execinfo.h would fail by default because
-I/usr/local/include was not being passed to the compiler when making the
checks for the header's existence.

Now that very recent FreeBSD versions (ie. 10-CURRENT) have NetBSD's
libexecinfo in base (and it is thus installed into /usr), the
backtrace-related checks would pass, but the required headers were not being
included in SystemInformation.cxx.

Change-Id: I3b91ed7ac0e6878035aee202b3336c536cc6d2ff