rpmsg: smd: Add missing include of sizes.h

Add missing include of sizes.h.

drivers/rpmsg/qcom_smd.c: In function ‘qcom_smd_channel_open’:
drivers/rpmsg/qcom_smd.c:809:36: error: ‘SZ_4K’ undeclared (first use in this function)
  bb_size = min(channel->fifo_size, SZ_4K);
                                    ^~~~~

Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>