oebb: work with bitbake master

oebb: work with bitbake master

Bitbake master doesn't enable fetch2 by default resulting in:

Parsing recipes...ERROR: Error evaluating '${@bb.fetch2.get_srcrev(d)}'
Traceback (most recent call last):
  File "/work/openembedded-core/sources/bitbake/lib/bb/data_smart.py", line 117, in expandWithRefs
    s = __expand_python_regexp__.sub(varparse.python_sub, s)
  File "/work/openembedded-core/sources/bitbake/lib/bb/data_smart.py", line 76, in python_sub
    value = utils.better_eval(codeobj, DataContext(self.d))
  File "/work/openembedded-core/sources/bitbake/lib/bb/utils.py", line 387, in better_eval
    return eval(source, _context, locals)
  File "SRCPV", line 1, in <module>
AttributeError: 'module' object has no attribute 'fetch2'

This fixes that

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>