sandbox: fix spl_board_init

sandbox_spl_defconfig with CONFIG_SPL_UNIT_TEST=n fails to build.

    in function `spl_board_init':
    arch/sandbox/cpu/spl.c:134:(.text.spl_board_init+0x4a):
    undefined reference to `ut_run_list'

Add the missing configuration check.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/spl.c b/arch/sandbox/cpu/spl.c
index 09e3d10..7590f1b 100644
--- a/arch/sandbox/cpu/spl.c
+++ b/arch/sandbox/cpu/spl.c
@@ -126,6 +126,9 @@
 {
 	struct sandbox_state *state = state_get_current();
 
+	if (!CONFIG_IS_ENABLED(UNIT_TEST))
+		return;
+
 	if (state->run_unittests) {
 		struct unit_test *tests = UNIT_TEST_ALL_START();
 		const int count = UNIT_TEST_ALL_COUNT();