staging: android: lowmemorykiller: well-marked debug print

Add "lowmemorykiller:" prefix to the debug print so it's easier to analyse LMK's debug output:
dmesg | grep lowmemorykiller

Signed-off-by: Dmitry Voytik <dvv.kernel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>