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>

drivers/rpmsg/qcom_smd.c

@@ -14,6 +14,7 @@
 #include <linux/platform_device.h>
 #include <linux/regmap.h>
 #include <linux/sched.h>
+#include <linux/sizes.h>
 #include <linux/slab.h>
 #include <linux/soc/qcom/smem.h>
 #include <linux/wait.h>