liblog: cleanup log_read.h header

There's no need to pull in <fcntl.h> to get O_NONBLOCK, since the
ANDROID_LOG_* macros are better standalone.

Also note that 0x00000003 should be reserved since these corresponded
to previously used values.

Test: build
Change-Id: Icaee6ee0250fe97d8324b3b340f7e196767b2711