fix: typo in `string(JOIN)` workaround (#12698)

My sketch to fix #12672 was wrong.  This works for realsies.

Closes #12698

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12698 from coryan:fix-cmake-typo-in-string-join-workaround 182d2e248e
PiperOrigin-RevId: 530116824

cmake/install.cmake

@@ -8,7 +8,7 @@ list(APPEND _pc_targets "utf8_range")
 
 set(_protobuf_PC_REQUIRES "")
 set(_sep "")
-foreach (_target IN LISTS _pc_target_list)
+foreach (_target IN LISTS _pc_targets)
   string(CONCAT _protobuf_PC_REQUIRES "${_protobuf_PC_REQUIRES}" "${_sep}" "${_target}")
   set(_sep " ")
 endforeach ()