Get rid of other-OS headers in .depend

Get rid of other-OS headers in .depend

It looks like blktrace_api.h is being parsed through the depend rule
in Makefile.[FreeBSD|NetBSD|mac] due to the headers being listed in
$(SOURCE) - gcc just ignores missing <> headers while clang generates an
error as it builds the dependency list. Since headers which are
included within source files will be listed as a dependency anyway, I
think they can be removed from $(SOURCE).

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>