um: Neaten vu_err macro definition

Defining a macro with ... and __VA_ARGS__ (without ##) can cause
compilation errors if a macro use does not have additional args.

Add ## to __VA_ARGS__ in the macro definition.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Richard Weinberger <richard@nod.at>