Cocoa: Fix build failure on OS X 10.11

The NSWindowCollectionBehaviorFullScreenNone enum value is missing from
system headers on 10.11 despite the documentation claiming it was added
in 10.7.

Unfortunately Apple has taken down all API release notes for versions
prior to 10.14.

This fixes a build failure introduced with
98d6e8485b.

src/cocoa_window.m

@@ -31,6 +31,10 @@
 #include <float.h>
 #include <string.h>
 
+// HACK: This enum value is missing from framework headers on OS X 10.11 despite
+//       having been (according to documentation) added in Mac OS X 10.7
+#define NSWindowCollectionBehaviorFullScreenNone (1 << 9)
+
 // Returns whether the cursor is in the content area of the specified window
 //
 static GLFWbool cursorInContentArea(_GLFWwindow* window)