Add sys/cdefs.h to log/log_main.h.

for liblog.vendor. __ANDROID_API__ is __ANDROID_API_FUTURE__
with BOARD_VNDK_VERSION=current, so we need it defined.

Bug: 33241851
Test: BOARD_VNDK_VERSION=current m libgui.vendor
      (that uses liblog.vendor)

Change-Id: I340ec048094c027828f516d891250651e0c88eea