spa: drop the pre-C99 checks from the log.h header

spa_log_logv was missing, so with this ifdef'd out we already fail to
compile:
spa/plugins/alsa/alsa-acp-device.c:1003: undefined reference to `spa_log_logv'

And it appears that the only requirement is __VA_ARGS__ support in the
compiler which we require in other places anyway.