FIX: Add print_banner to required functions validation

CRITICAL BUG FIX: print_banner was being called in show_scan_menu but was not
listed as a required function in the validation check. If the common-functions.sh
library failed to source properly, print_banner would be undefined, causing the
menu to fail with 'command not found' error.

Changes:
- Added 'print_banner' to the list of required functions validated at startup
- This ensures print_banner is available before attempting to use it
- Script now fails early with clear error message if library is missing

This prevents silent failures when the menu tries to display.

modules/security/malware-scanner.sh

@@ -34,6 +34,7 @@ validate_required_functions() {
     local required_functions=(
         "confirm"
         "print_header"
+        "print_banner"
         "select_user_interactive"
         "get_user_domains"
     )