sdei: include "context.h" to fix compilation errors

This patch includes context.h from sdei_private.h to fix the
following compilation errors:

<snip>
In file included from services/std_svc/sdei/sdei_event.c:9:0:
services/std_svc/sdei/sdei_private.h: In function 'sdei_client_el':
services/std_svc/sdei/sdei_private.h:164:2: error: unknown type name 'cpu_context_t'
  cpu_context_t *ns_ctx = cm_get_context(NON_SECURE);
  ^
services/std_svc/sdei/sdei_private.h:165:2: error: unknown type name 'el3_state_t'
  el3_state_t *el3_ctx = get_el3state_ctx(ns_ctx);
  ^
services/std_svc/sdei/sdei_private.h:165:2: error: implicit declaration of function 'get_el3state_ctx' [-Werror=implicit-function-declaration]
services/std_svc/sdei/sdei_private.h:165:25: error: initialization makes pointer from integer without a cast [-Werror]
  el3_state_t *el3_ctx = get_el3state_ctx(ns_ctx);
                         ^
services/std_svc/sdei/sdei_private.h:167:2: error: implicit declaration of function 'read_ctx_reg' [-Werror=implicit-f...

services/std_svc/sdei/sdei_private.h

@@ -8,6 +8,7 @@
 #define SDEI_PRIVATE_H
 
 #include <arch_helpers.h>
+#include <context.h>
 #include <context_mgmt.h>
 #include <debug.h>
 #include <errno.h>