defconfig: sdm855: Disable DEBUG_OBJECTS_RCU_HEAD to save memory

DEBUG_OBJECTS_RCU_HEAD has significant memory usage. In
cases of stress testing, where RCU gets excessive usage,
these debug objects increase memory pressure, and yields
false positive memory issues. Disable by default.

Change-Id: I31828ab5aff93cc1ca71b6e5174d4ccf8d840b73
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>