build: make vo_sdl conflict with cocoa

These are inherently incompatible. As far as I'm aware, SDL must be used
from the main thread on OSX.

(Technically, this condition is wrong: the problem happens on OSX in
general, or more precisely, when SDL uses Cocoa. I didn't find the waf
OSX dependency name after 5 second of searching, so I'm just using
cocoa, without which mpv is useless on OSX anyway.)

wscript

@@ -576,6 +576,7 @@ video_output_features = [
         'name': '--sdl2-video',
         'desc': 'SDL2 video output',
         'deps': 'sdl2',
+        'deps_neg': 'cocoa',
         'func': check_true,
     }, {
         'name': '--cocoa',