cocoa: Fix warnings about invalid prototype declarations

cocoa: Fix warnings about invalid prototype declarations

Fix the following Cocoa trivial warnings:

  C compiler for the host machine: cc (clang 14.0.0 "Apple clang version 14.0.0 (clang-1400.0.29.202)")
  Objective-C compiler for the host machine: clang (clang 14.0.0)

  [100/334] Compiling Objective-C object libcommon.fa.p/net_vmnet-bridged.m.o
  net/vmnet-bridged.m:40:31: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  static char* get_valid_ifnames()
                                ^
                                 void
  [742/1436] Compiling Objective-C object libcommon.fa.p/ui_cocoa.m.o
  ui/cocoa.m:1937:22: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  static int cocoa_main()
                       ^
                        void

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230425192820.34063-1-philmd@linaro.org>