test: missing dependency for test/cmd/setexpr.c

test/cmd/setexpr.c cannot be linked with CONFIG_CMD_SETEXPR=n:

ld.bfd: test/built-in.o: in function `setexpr_test_sub':
test/cmd/setexpr.c:227: undefined reference to `setexpr_regex_sub'
ld.bfd: test/built-in.o: in function `setexpr_test_backref':
test/cmd/setexpr.c:267: undefined reference to `setexpr_regex_sub'

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/cmd/Makefile b/test/cmd/Makefile
index 5451e9e..c84df60 100644
--- a/test/cmd/Makefile
+++ b/test/cmd/Makefile
@@ -8,4 +8,4 @@
 obj-y += mem.o
 obj-$(CONFIG_CMD_MEM_SEARCH) += mem_search.o
 obj-$(CONFIG_CMD_PWM) += pwm.o
-obj-y += setexpr.o
+obj-$(CONFIG_CMD_SETEXPR) += setexpr.o