qdoc: allow 'void' function parameter without a parameter name

There are a few functions using a C-style function declaration:

    void foo(void);

meaning that foo() takes no parameters. This change allows this
for QDoc, making it successfully match documented \fn blocks
with the correct declaration, and not print out warnings.

Change-Id: I8191c55094371431b0e9c2ad22d19cadcb7facfb
Reviewed-by: Martin Smith <martin.smith@digia.com>