CPack/RPM: Append .rpm to CPACK_RPM_FILE_NAME if missing

Previously we issued an error when the `.rpm` suffix is missing.
Instead, append the suffix automatically.  This matches the behavior of
`CPACK_ARCHIVE_FILE_NAME`, to which the archive format suffix is
automatically appended.  With this change, developers can simply do

    set(CPACK_RPM_comp_FILE_NAME "${CPACK_ARCHIVE_comp_FILE_NAME}")
7 files changed