Fix typing rule for Concat.
Fix typing rule for Concat.

mrakgr/futhark,diku-dk/futhark,diku-dk/futhark,diku-dk/futhark,mrakgr/futhark,mrakgr/futhark,HIPERFIT/futhark,ihc/futhark,ihc/futhark,HIPERFIT/futhark,diku-dk/futhark,HIPERFIT/futhark,CulpaBS/wbBach,diku-dk/futhark,ihc/futhark