I18N: Add type declaration to new method missed in [57518].

See #59656.

git-svn-id: https://develop.svn.wordpress.org/trunk@57519 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/l10n/class-wp-translation-file.php

@@ -247,7 +247,7 @@ public function get_plural_form( int $number ): int {
 	 * @param string $header Plural-Forms header string.
 	 * @return string Plural forms expression.
 	 */
-	protected function get_plural_expression_from_header( $header ) {
+	protected function get_plural_expression_from_header( string $header ): string {
 		if ( preg_match( '/^\s*nplurals\s*=\s*(\d+)\s*;\s+plural\s*=\s*(.+)$/', $header, $matches ) ) {
 			return trim( $matches[2] );
 		}