dwrite.h: Add missed parenthesis

This commit partially fixes compilation of a C code module which
#include's dwrite.h.  One still can't compile such modules successfully
because some classes in dwrite.h declare overloaded member functions.

No functional changes intended.

Signed-off-by: ‡Ruslan Garipov <ruslanngaripov@gmail.com>
Signed-off-by: Liu Hao <lh_mouse@126.com>