ui/cocoa: Fix openFile: deprecation on Big Sur

ui/cocoa: Fix openFile: deprecation on Big Sur

ui/cocoa.m:1188:44: warning: 'openFile:' is deprecated: first deprecated in macOS 11.0 - Use -[NSWorkspace openURL:] instead.
      [-Wdeprecated-declarations]
        if ([[NSWorkspace sharedWorkspace] openFile: full_file_path] == YES) {
                                           ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/AppKit.framework/Headers/NSWorkspace.h:350:1: note:
      'openFile:' has been explicitly marked deprecated here
- (BOOL)openFile:(NSString *)fullPath API_DEPRECATED("Use -[NSWorkspace openURL:] instead.", macos(10.0, 11.0));
^

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210102150718.47618-1-r.bolshakov@yadro.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>