rpmsg: glink: add include of header file

With an x86_64 architecture W=1 build, qcom_glink_ssr_notify() is
reported as having no previous prototype.  The prototype is found in
"qcom_glink.h", so we just need "qcom_glink_ssr.c" to include that
file.
Signed-off-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20210105235528.32538-1-elder@linaro.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/rpmsg/qcom_glink_ssr.c

@@ -8,6 +8,7 @@
 #include <linux/module.h>
 #include <linux/notifier.h>
 #include <linux/rpmsg.h>
+#include <linux/rpmsg/qcom_glink.h>
 #include <linux/remoteproc/qcom_rproc.h>
 
 /**