iOS: Look up OpenGL symbols using RTLD_DEFAULT instead of RTLD_NEXT

Even if the call site is in the application binary, it's safer to use
RTLD_DEFAULT as that implies using the default library search order.

Change-Id: I1b30bded92b95fc7451fcdbf7afd7444dcecea71
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>