Unbreak build with libv4l-0.9.1 or later

Unmask libv4l2.h for default pragma visibility in Mozilla build.
Since 50bde45 libv4l no longer alters visibility for consumers.
And HAVE_VISIBILITY is defined only for internal or gnulib build.

http://git.linuxtv.org/cgit.cgi/v4l-utils.git/commit/?id=50bde45
http://git.savannah.gnu.org/cgit/gnulib.git/tree/m4/visibility.m4

Noticed by: kwm (via review D1482)