incr.comp.: Fix build issue in rustc_incremental if CFG_VERSION is not set.
incr.comp.: Fix build issue in rustc_incremental if CFG_VERSION is not set.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust