Android: fix build break from r600/radeon split

Android: fix build break from r600/radeon split

Commit 06bfb2d28f7a ("r600: fork and import gallium/radeon") broke the
Android build:

external/mesa3d/src/gallium/drivers/radeon/r600_pipe_common.c:43:10: fatal error: 'llvm-c/TargetMachine.h' file not found
         ^~~~~~~~~~~~~~~~~~~~~~~~

Update the Android makefiles so that drivers/radeon is only built when
radeonsi (and therefore LLVM) is enabled.

Fixes: 06bfb2d28f7a (r600: fork and import gallium/radeon)
Acked-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Rob Herring <robh@kernel.org>