IE compatibility meta is missing in overall_header.html

For some reason IE compatibility meta was removed from 3.1 during development in this commit: https://github.com/phpbb/phpbb/commit/854c14f9f6ae78318e159e27724178579ff48dcc

Looks like reason for removal was not being able to validate that tag. Validation doesn't matter. Tag is not invalid, that meta is needed for IE users with broken configuration.