Genex: Fix TARGET_PROPERTY value of SOURCES

Refactoring in commit v3.8.0-rc1~445^2~2 (cmTarget: Move sanity checks
and computed property access to callers, 2016-10-13) exposed a typo in
commit v3.8.0-rc1~445^2~3 (cmGeneratorTarget: Implement
cmTargetPropertyComputer interface, 2016-10-13).  Together they broke
the `$<TARGET_PROPERTY:mytgt,SOURCES>` generator expression in the case
that the `SOURCES` target property is populated in part by the
`target_sources` command.  Add the missing `;`-separator.

Fixes: #17243
Status Job ID Name Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager