android: util: add log.c to Makefile.sources

android: util: add log.c to Makefile.sources

Fixes the following building errors:

external/mesa/src/intel/vulkan/anv_android.c:627: error: undefined reference to 'mesa_log'
...
external/mesa/src/intel/vulkan/anv_device.c:164: error: undefined reference to 'mesa_log'

Fixes: 13ea7db76 ("mesa: Promote Intel's simple logging façade for Android to util/")
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6972>