qdoc: Fix handling of '#if defined' preprocessor statements

Fix the regular expression that incorrectly made parentheses
mandatory for the #if condition; an optional specifier ('?')
was missing for the closing parenthesis.

Task-number: QTBUG-62931
Change-Id: Ia046d4217f64560dd49707ec2d01b73071ba2d03
Reviewed-by: Martin Smith <martin.smith@qt.io>