build: ignore deprecated-declaration warnings in objc++ macOS code

These come from GUI code, and haven't/aren't being fixed, see discussion
in https://github.com/bitcoin-core/gui/issues/112. For now, just ignore
them entirely. Note that this only applies to ObjCXX code, so will not
hide any relevant warnings coming from C or CXX code (and they would be
unlikely in any case).

Alternative to #29362, which disables all compiler warnings, for macOS
builds in the CI.

Relevant output:
```bash
qt/macnotificationhandler.mm:27:9: warning: 'NSUserNotification' is deprecated: first deprecated in macOS 11.0 - All NSUserNotifications API should be replaced with UserNotifications.frameworks API [-Wdeprecated-declarations]
        NSUserNotification* userNotification = [[NSUserNotification alloc] init];
        ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSUserNotification.h:24:12: note: 'NSUserNotification' has been explicitly marked deprecated here
@interface NSUserNotification : NSObject <NSCopying> {
           ^
qt/macnotificationhandler.mm:27:50: warning: 'NSUserNotification' is deprecated: first deprecated in macOS 11.0 - All NSUserNotifications API should be replaced with UserNotifications.frameworks API [-Wdeprecated-declarations]
        NSUserNotification* userNotification = [[NSUserNotification alloc] init];
                                                 ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSUserNotification.h:24:12: note: 'NSUserNotification' has been explicitly marked deprecated here
@interface NSUserNotification : NSObject <NSCopying> {
           ^
qt/macnotificationhandler.mm:30:11: warning: 'NSUserNotificationCenter' is deprecated: first deprecated in macOS 11.0 - All NSUserNotifications API should be replaced with UserNotifications.frameworks API [-Wdeprecated-declarations]
        [[NSUserNotificationCenter defaultUserNotificationCenter] deliverNotification: userNotification];
          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Foundation.framework/Headers/NSUserNotification.h:118:12: note: 'NSUserNotificationCenter' has been explicitly marked deprecated here
@interface NSUserNotificationCenter : NSObject {
           ^
3 warnings generated.
```

0 files changed, 0 insertions, 0 deletions