storage_backend_rbd.C: Fix return type of a volStorageBackendRBDUseFastDiff() stub

storage_backend_rbd.C: Fix return type of a volStorageBackendRBDUseFastDiff() stub

Inside of storage_backend.c there are two implementations of
volStorageBackendRBDUseFastDiff() function: one when librbd is
new enough and one when it isn't. The former returns a bool, but
the latter is declared to return an int despite it returning a
boolean. Fix the latter.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>