ANDROID: kernel/Makefile: do not disable LTO for sys_ni.c with CFI

sys_ni.c compiles fine with CONFIG_LTO_CLANG, and disabling LTO
for it breaks indirect call checking to functions in this file.

Bug: 138254717
Change-Id: I7947cf3d0283ad37431860739665fee7fb0dfbdb
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>