UI: Remove unused header for macOS

UI/platform-osx.mm

@@ -24,7 +24,6 @@
 #include "obs-app.hpp"
 
 #include <unistd.h>
-#include <sys/sysctl.h>
 
 #import <AppKit/AppKit.h>
 #import <CoreFoundation/CoreFoundation.h>