CPack/DEB: Avoid overriding user-provided `postinst` and `postrm`

When a user provides `CPACK_DEBIAN_PACKAGE_CONTROL_EXTRA` or
`CPACK_DEBIAN_<comp>_PACKAGE_CONTROL_EXTRA` variables in
`CMakeLists.txt` and the package contains dynamic libraries, the
`CPackDeb.cmake` sets `CPACK_ADD_LDCONFIG_CALL` to `1`.  Later it
analyzes if defaulted `postinst`/`postrm` should be generated trying to
check if the user provides any in `CPACK_DEBIAN_PACKAGE_CONTROL_EXTRA`
variable. However, the `foreach` loop uses the invalid variable
`PACKAGE_CONTROL_EXTRA` instead, so these files gonna be overridden.
Fix the variable name.

Fixes: #22410