sandbox: Don't include the main loop in SPL

SPL does not have a command interface so we should not include the main loop
code.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/start.c b/arch/sandbox/cpu/start.c
index 969618e..6e4ec01 100644
--- a/arch/sandbox/cpu/start.c
+++ b/arch/sandbox/cpu/start.c
@@ -73,6 +73,7 @@
 }
 SANDBOX_CMDLINE_OPT_SHORT(help, 'h', 0, "Display help");
 
+#ifndef CONFIG_SPL_BUILD
 int sandbox_main_loop_init(void)
 {
 	struct sandbox_state *state = state_get_current();
@@ -97,6 +98,7 @@
 
 	return 0;
 }
+#endif
 
 static int sandbox_cmdline_cb_boot(struct sandbox_state *state,
 				      const char *arg)