osd: fix PG dtor compile error

osd: fix PG dtor compile error

We need at least none non-pure virtual method to tell gcc where the
vtable goes.  The destructor wins!

libosd.a(libosd_a-ReplicatedPG.o): In function `~PG':
/home/sage/src/ceph/src/osd/PG.h:1367: undefined reference to `vtable for PG'
libosd.a(libosd_a-ReplicatedPG.o):(.rodata._ZTI12ReplicatedPG[typeinfo for ReplicatedPG]+0x10): undefined reference to `typeinfo for PG'
libosd.a(libosd_a-PG.o): In function `PG':
/home/sage/src/ceph/src/osd/PG.cc:85: undefined reference to `vtable for PG'
...

Signed-off-by: Sage Weil <sage@inktank.com>