gbm: work around mesa linkage issue

Mesa drivers such as i965_dri.so use symbols from libglapi but do not
pull in libglapi (https://bugs.freedesktop.org/show_bug.cgi?id=57702)
so dlopen libglapi before we need it.

Fixes #21: https://github.com/waffle-gl/waffle/pull/21
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>