ANDROID: uid_sys_stats: Fix implicit declaration of get_cmdline()

Include linux/mm.h for get_cmdline() declaration.

Change-Id: Icad6ef7deef4d93d92d423c96bfa61fb5d66d0b7
Fixes: Change-Id: I30083b757eaef8c61e55a213a883ce8d0c9cf2b1
       ("uid_sys_stats: log task io with a debug flag")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>