Fix underlinking in libOSMesa since commit adefee5 "Always build shared glapi"

Fix underlinking in libOSMesa since commit adefee5 "Always build shared glapi"

Since we now always build shared glapi, this exposes the fact that libOSMesa was
underlinked when glapi was built shared.

Fix this by doing the same thing as drivers/X11/Makefile already does, ensuring
that the library is linked with the shared glapi library.

(I'm not clear why we link with both glapi.a and glapi.so, so this may be all wrong)

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>