ui/cocoa.m compile error

[Liviu Ionescu]

Hi,

I'm building 6.2.0 on macOS, and on a recent macOS 11.6 with Apple Silicon the 
build passes, but on a slightly older macOS 10.13, which is my base platform 
for Intel macOS builds, compiling ui/cocoa.m fails:

```
../../sources/qemu-6.2.0.git/ui/cocoa.m:1766:52: error: unknown type name 
'NSPasteboardTypeOwner'; did you mean 'NSPasteboardType'?

@interface QemuCocoaPasteboardTypeOwner : NSObject<NSPasteboardTypeOwner>
                                                   ^~~~~~~~~~~~~~~~~~~~~
                                                   NSPasteboardType

/System/Library/Frameworks/AppKit.framework/Headers/NSPasteboard.h:22:20: note: 
'NSPasteboardType' declared here
typedef NSString * NSPasteboardType NS_EXTENSIBLE_STRING_ENUM;
                   ^
../../sources/qemu-6.2.0.git/ui/cocoa.m:1766:43: error: type arguments cannot 
be applied to non-parameterized class 'NSObject'

@interface QemuCocoaPasteboardTypeOwner : NSObject<NSPasteboardTypeOwner>
                                          ^       ~~~~~~~~~~~~~~~~~~~~~~~

3 warnings and 2 errors generated.
```

I checked the Git history and the cocoa.m:1766 line is part of commit 7e3e20d8 
from 23 Jun 2021:

- 
https://github.com/xpack-dev-tools/qemu/commit/7e3e20d89129614f4a7b2451fe321cc6ccca3b76

I don't have experience with Cocoa programming, but apparently this recent 
change is not compatible with older Macs.

Any suggestions on how to approach this?

Thank you,

Liviu