build: unbreak build for osx

We have CI! And even through our Infra is dead and too stupid to use it,
you can push your commits there before. This means *you can test* if you
build "fixes" are fixing all platforms, or break for example osx. *This
one* brings osx back.

<3 CI!

Reviewed-by: Lauro Neto <lauromauro_>
Differential Revision: https://phab.enlightenment.org/D8097

header_checks/meson.build

@@ -208,6 +208,9 @@ if sys_bsd == true
   config_h.set('HAVE_NOTIFY_KEVENT', '1')
 endif
 
+if sys_osx == true
+  config_h.set('HAVE_NOTIFY_COCOA', '1')
+endif
 config_h.set('SIZEOF_INT', cc.sizeof('int'))
 config_h.set('SIZEOF_LONG', cc.sizeof('long'))