cmake: disable oss by default on OpenBSD, not FreeBSD

This fixes ed3fad18808714f9fab3111a45d06264ea6fb0c5

CMakeLists.txt

@@ -220,7 +220,7 @@ if(VITA OR PSP OR PS2 OR N3DS OR RISCOS)
   set(SDL_LOADSO_DEFAULT OFF)
 endif()
 
-if((RISCOS OR UNIX_SYS) AND NOT (LINUX OR NETBSD OR FREEBSD))
+if((RISCOS OR UNIX_SYS) AND NOT (LINUX OR NETBSD OR OPENBSD))
   set(SDL_OSS_DEFAULT ON)
 else()
   set(SDL_OSS_DEFAULT OFF)