libbinder: use pragma once

Instead of copying a file and changing one header guard, I'd rather
change everything to pragma once, so I don't have to mess with them.
This has other benefits, like avoiding adding extra defines, which
people might depend on.

Bug: 167966510
Test: builds
Change-Id: Ia03ecb2edd0dc8ef3feb9a2594c1e602106268c3