Move {is,load}_blktrace() to a new header blktrace.h

Move {is,load}_blktrace() to a new header blktrace.h

This commit does exactly the same as what cgroup.h does for cgroup
support.

Since blktrace is Linux specific feature and api compiled only on
Linux and Android, these two could be defined in blktrace.h rather
than in os/os.h which is basically intended to be something generic
whose implementation vary depending on supported platforms.

The reason for adding a new header blktrace.h instead of using
blktrace_api.h is because blktrace_api.h seems to have been added
for blktrace.c to include which gets compiled only on Linux where
blktrace apis are basically guaranteed to exist (unless really old
kernels), whereas these prototypes or inline functions are needed
by all platforms.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>