When qcom_smd is a loadable module and wcnss-pil is built-in,
we get a link error:

drivers/remoteproc/qcom_wcnss_pil.o: In function `wcnss_smd_remove':
qcom_wcnss_iris.c:(.text.wcnss_smd_remove+0x10): undefined reference to `qcom_smd_unregister_edge'
drivers/remoteproc/qcom_wcnss_pil.o: In function `wcnss_smd_probe':
qcom_wcnss_iris.c:(.text.wcnss_smd_probe+0x12): undefined reference to `qcom_smd_register_edge'

This adds a Kconfig dependency to avoid this. We can still allow
build-testing with SMD disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>