Hide gnu extension warnings in log/log.h

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

log/log.h uses gnu extensions for zero-argument variadic macro token
pasting, which clang warns about with -Weverything.  Hide the warnings
using pragmas.

Test: m -j native
Bug: 31492149
Change-Id: Ib2113f877d811a6ecc0e91c035640b506f04bd86