cocoa: always compile OSX application code with cocoa

This unbreaks compiling command line player and libmpv at the same
time. The problem was that doing so silently disabled the OSX
application thing - but the command line player can not use the
vo_opengl Cocoa backend without it.

The OSX application code is basically dead in libmpv, but it's not
that much code anyway.

If you want a mpv binary that does not create an OSX application
singleton (and creates a menu etc.), you must disable cocoa
completely, as cocoa can't be used anyway in this case.

osdep/macosx_application_objc.h

@@ -27,5 +27,3 @@
 @property(nonatomic, retain) NSArray *files;
 @property(nonatomic, assign) size_t openCount;
 @end
-
-Application *mpv_shared_app(void);