staging: greybus: fix more header declarations

More headers needed to be fixed when moving greybus out of staging and
enabling the COMPILE_TEST option.

Add forward declarations for the needed structures.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190828124825.20800-1-rui.silva@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/greybus/operation.h

@@ -15,7 +15,7 @@
 #include <linux/types.h>
 #include <linux/workqueue.h>
 
-
+struct gb_host_device;
 struct gb_operation;
 
 /* The default amount of time a request is given to complete */

include/linux/greybus/svc.h

@@ -12,6 +12,8 @@
 #include <linux/types.h>
 #include <linux/device.h>
 
+struct gb_svc_l2_timer_cfg;
+
 #define GB_SVC_CPORT_FLAG_E2EFC		BIT(0)
 #define GB_SVC_CPORT_FLAG_CSD_N		BIT(1)
 #define GB_SVC_CPORT_FLAG_CSV_N		BIT(2)