fix parallel build of btt and blkiomon

fix parallel build of btt and blkiomon

rbtree.c is used by both binaries. It is possible that when make -C btt
is invoked rbtree.o does not exist yet, but is already schedule by the
compilation of blkiomon. That could result in recompiling rbtree.o again
for btt/btt.
In that case, at install time, make will recompile blkiomon which can
fail in gentoo, because CC variable is not overriden by ebuild script at
install time. (see https://bugs.gentoo.org/705594)

Add a dependency on SUBDIRS to wait for all binary in . to be compiled.
It will guarante rbtree.o exists.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>