Fix warnings in cutils/atomic.h

system/core/include is included in the global include path using
-isystem, which hides all warnings.  Fix warnings in cutils/atomic.h in
preparation for moving from -isystem to -I.

Test: m -j native
Bug: 31492149
Change-Id: I2a212f192309090a16d507e9ab6acf0fd7cac02d