sandbox: Correct SDL build flags

The check for CONFIG_SANDBOX_SDL in config.mk does not work since the
build config is not available by the time that file is included. Remove it
so that we always call sdl-config except when NO_SDL is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/sdl.c b/arch/sandbox/cpu/sdl.c
index f668f53..080c7c8 100644
--- a/arch/sandbox/cpu/sdl.c
+++ b/arch/sandbox/cpu/sdl.c
@@ -6,7 +6,7 @@
 #include <errno.h>
 #include <unistd.h>
 #include <linux/input.h>
-#include <SDL/SDL.h>
+#include <SDL.h>
 #include <asm/state.h>
 
 /**