cmTarget: Fix CMP0022 OLD breakage from recent refactoring

In commit 7b0834e9 (cmTarget: Refactor internal LinkImplementation map,
2014-06-19) cmTarget::GetLinkImplementationLibrariesInternal was changed
accidentally to pass "this" to ComputeLinkImplementation instead of
"head".  Change it back.
Merge topic 'refactor-link-internals' into next

f45ede61 cmTarget: Fix CMP0022 OLD breakage from recent refactoring