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>