default.xml: Bump bitbake revision

Bump bitbake revision to include bb.build.clean_stamps() which is needed
by oe_core staging.bbclass

* 494fcfcb (HEAD -> 2.0, origin/2.0) build: Add clean_stamp API function to allow removal of task stamps - Richard Purdie
* 943701ee data: Do not depend on vardepvalueexclude flag - Tomasz Dziendzielski
* 8ac6e094 fetch2/osc: Small fixes for osc fetcher - Gunjan Gupta

Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>