Bump xcconfigs to remove USER_HEADER_SEARCH_PATHS

See https://github.com/jspahrsummers/xcconfigs/pull/31.
https://github.com/Quick/Nimble/compare/81a2d8a63083ae6512d40f7a02d5e075e57be317...6f787eeb75b2fa71464981f28b3dc8d7bd532e69