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

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-function-declaration]
  return ((read_ctx_reg(el3_ctx, CTX_SCR_EL3) & SCR_HCE_BIT) != 0U) ?
  ^
services/std_svc/sdei/sdei_private.h:167:33: error: 'CTX_SCR_EL3' undeclared (first use in this function)
  return ((read_ctx_reg(el3_ctx, CTX_SCR_EL3) & SCR_HCE_BIT) != 0U) ?
                                 ^
services/std_svc/sdei/sdei_private.h:167:33: note: each undeclared identifier is reported only once for each function it appears in
cc1: all warnings being treated as errors
<snip>

Change-Id: Id0cad56accf81b19cb0d301784f3f086dd052722
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>