Avoid mesa_dri_drivers import lib being installed

On Cygwin and MinGW, linking a shared library also generates an import library

Use a wildcard which also matches the name of the megadriver import lib,
mesa_dri_drivers.dll.a, so that is also removed after megadriver symlinks are
created

(This then matches src/gallium/targets/dri/Makefile.am, which already does
things this way)

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>