build: force .so extension for the gallium dri modules

build: force .so extension for the gallium dri modules

While linux uses .so as a default extension for shared libraries that is
not the case for other platforms. The loader in libGL (and others) assumes
that the dri module will always have a .so extension, thus it will fail
to load on the affected platforms.

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