url: Make sure Haiku's C++ implementation is flagged as 'extern "C"'

src/misc/haiku/SDL_sysurl.cc

@@ -23,6 +23,8 @@
 #include "SDL_error.h"
 #include <Url.h>
 
+extern "C" { int SDL_SYS_OpenURL(const char *url); }
+
 int
 SDL_SYS_OpenURL(const char *url)
 {