ANDROID: staging: ion: fix sparse warning in ion system heap

drivers/staging/android/ion/heaps/ion_system_heap.c:250:24: sparse: sparse:
symbol 'system_heap' was not declared. Should it be static?

Fixes: 142434553
Test: Build and boot cuttlefish

Change-Id: I6e1eb9365ca12c2d081533ed2db9881086531ec7
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sandeep Patil <sspatil@google.com>