.gitignore: libbacktrace.a now libbacktracevogl.a

Somewhere along the line, libbacktrace.a, previously ignored, changed
names to libbacktracevogl.a

Signed-off-by: Lawrence L Love <lawrencex.l.love@intel.com>
Merge pull request #48 from lawlove/gitignore

.gitignore: libbacktrace.a now libbacktracevogl.a