meson: Build libdrm.so as an unversioned lib on Android.

Android vendor libraries don't have sonames, and libdrm.so shouldn't
either.  This lets a Mesa built against a libdrm.so built for Android
be copied directly to a Chrome OS ARC installation.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>