make: fix with_rbd guard

make: fix with_rbd guard

FileJournal needs stuff from blkdev.c in libcommon.

./.libs/libosd.a(libos_la-FileJournal.o): In function `FileJournal::_open_block_device()':
/home/nwatkins/src/ceph/src/os/FileJournal.cc:139: undefined reference to `get_block_device_size(int, long*)'
/home/nwatkins/src/ceph/src/os/FileJournal.cc:161: undefined reference to `block_device_support_discard(char const*)'
./.libs/libosd.a(libos_la-FileJournal.o): In function `FileJournal::do_discard(long, long)':
/home/nwatkins/src/ceph/src/os/FileJournal.cc:1587: undefined reference to `block_device_discard(int, long, long)'

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>