fix build error 'strlen' is not a member of 'std' locally

Change-Id: I587ef2c38fd9377eef2073a49cd8497b2bd4bf9f
Reviewed-on: https://gerrit.libreoffice.org/10076
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>