[sonic-swss-common]: Update Submodule

Update in thie change:
640a218 [packaging]: Add Support For Libboost v1.71.0 (sonic-net#449)
2f79bb4 Fix ConfigDBPipeConnector.get_config() implementation (sonic-net#446)
fc64e3a [dbconnector] protect db information with mutex (sonic-net#423)
e8a03de [ci]: use sonicbld pool (sonic-net#445)

signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>