log: Fix warning with __USE_MINGW_ANSI_STDIO

I haven't seen this warning in any AOSP code, but some of our code is
using this header differently than the rest of the build system, and
hits this warning-turned-error:

error: "__USE_MINGW_ANSI_STDIO" is not defined [-Werror=undef]

Change-Id: I3c5a91f107af8bf6078e06f2ce326c5466349b79