egl_dri2: Export glapi symbols for DRI drivers.

egl_dri2: Export glapi symbols for DRI drivers.

When an app loads libEGL.so dynamically with RTLD_LOCAL, loading DRI
drivers would fail because of missing glapi symbols.  This commit makes
egl_dri2 load libglapi.so with RTLD_GLOBAL to export glapi symbols for
future symbol resolutions.

The same trick can be found in GLX.  However, egl_dri2 can only do so
when --enable-shared-glapi is given.  Because, otherwise, both libGL.so
and libglapi.so define glapi symbols and egl_dri2 cannot tell which
library to load.