android: radv: build vulkan.radv conditionally to radeonsi

A problem was reported with arm,arm64 targets build due to missing
libLLVM shared library dependency with AOSP; to avoid this issue vulkan.radv
is built conditionally only when radeonsi is in BOARD_GPU_DRIVERS

Fixes: 0ca153f8 ("android: radv: enable build of vulkan.radv HAL module")

Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: "18.2" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 1c7a2433)