meson: ensure i915 Gallium driver includes Intel sources
Ensure builds flag the use of Intel sources when the i915 Gallium driver
is configured (-Dgallium-drivers=i915
). Otherwise, a build may fail if
other Intel-based configuration options are not enabled:
./src/gallium/winsys/i915/drm/meson.build:21:0: ERROR: Unknown variable "libintel_common".
This resolves #7782 (closed).