ui/cocoa.m: eliminate normalWindow warning

Eliminate this warning associated with the setting of the normalWindow's title:

ui/cocoa.m: In function '-[QemuCocoaAppController init]':
ui/cocoa.m:888:9: warning: format not a string literal and no format arguments
 [-Wformat-security]
         [normalWindow setTitle:[NSString stringWithFormat:@"QEMU"]];

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 57057D6E-C108-4AE1-8370-E7E6855B2F2C@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
