ui/cocoa.m: Fix compatibility issue with Mac OS 10.9 and under

The [NSEvent modifierFlags] method returns an NSEventModifierFlags type value in Mac OS 10.10. It use to be of type NSUInteger. Replacing NSEventModifierFlags with NSUInteger allows for the cooca.m file to be compiled on older versions of Mac OS. This patch was been tested on Mac OS 10.6 and Mac OS 10.12 without problem.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: F6C36C1A-4661-48F4-BEA6-3994889927D0@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>