Palette headers missing in plugins

[sterborg]

Since the migration from list_type to CType the headers palette is missing in solr plugins showitem config.

In the commit the addition of 'pi_flexform' is replaced by the full definition. Since the headers palette is no part of the automatically added palettes it is left out.

ToDo: add "--palette--;;headers," to showitem definition.

TYPO3 13.4.18