build: include ax_gcc_func_attribute.m4

Some systems lack the helper, so ship a local copy of it. Pulled from Mesa, just like many other parts of virgl.

Fixes: 6564d139 ("virglrenderer: enable symbol visiblity.") Cc: Dave Airlie airlied@redhat.com Cc: Elie Tournier elie.tournier@collabora.com Reported-by: Elie Tournier elie.tournier@collabora.com Signed-off-by: Emil Velikov emil.velikov@collabora.com