The new OGL extension code included <SDL/SDL.h>, which works when the
SDL headers are installed in a directory named SDL and that directory is
in a system search directory. Those conditions usually hold on Linux,
both for /usr/include and /usr/local/include. However, non-Linux users
sometimes install SDL in other places. Kreator reports that the OGL
build on OS X failed because his SDL headers were on the search path,
but not in a directory named SDL on the search path. Switch the
include directive to an unqualified name to match other SDL includes.
Reported-by: kreatordxx <private mail>
Fixes: db267af6f2 ("add basic OpenGL extension handling")
