Kconfig: select RCU_BOOST if Kmemleak is enabled

ARCH_MSM8916 and ARCH_MSM8909 based targets have been observed
to queue up too many kmemleak RCU callbacks. This leads to timer
interrupt getting scheduled for every jiffy for an extended
period of time. We end up starving pending tasks causing
undesired effects like watchdog timeout.

Enable RCU_BOOST to ensure read side critical sections get over
quickly and thus ensuring callbacks get executed faster.

Change-Id: Idf38131a7231b69680d8e3bf7cb2b7f3711082bd
Signed-off-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>