sandbox: Add a prototype for cleanup_before_linux()

This function is defined but has no prototype declaration. Add it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/include/asm/u-boot-sandbox.h

@@ -23,4 +23,6 @@ int dram_init(void);
 int sandbox_early_getopt_check(void);
 int sandbox_main_loop_init(void);
 
+int cleanup_before_linux(void);
+
 #endif	/* _U_BOOT_SANDBOX_H_ */