sanity.bbclass: Add which to SANITY_REQUIRED_UTILITIES.
Using docker with the Fedora 23 container exposed an issue with
the rpm-native configure step. If which is not present the configure
script fork bombs. After much pain, I tracked this to which not being
present in the default container. Add a check for which so others
do not have to have this experience.
Signed-off-by: Philip Balister <philip@balister.org> Signed-off-by: Ross Burton <ross.burton@intel.com>
