hv:fix "missing for discarded return value" for memcpy_s and strcpy_s

It will print error information inside memcpy_s if
the parameteter is invalid, the caller can not check
the return value for memcpy_s/strcpy_s/strncpy_s
code like this:
int a(void) {
return 0;
}
int b(void){
a();
}
fix as follow:
int a(void) {
return 0;
}
int b(void){
(void)a();
}

Signed-off-by: Mingqiang Chi <mingqiang.chi@intel.com>

hypervisor/lib/crypto/hkdf.c

@@ -17,7 +17,7 @@ int hkdf_sha256(uint8_t *out_key, size_t out_len,
 	 * to derive multiple seeds in order to support multiple
 	 * AaaG/Trusty instances.
 	 */
-	memcpy_s(out_key, out_len, secret, secret_len);
+	(void)memcpy_s(out_key, out_len, secret, secret_len);
 
 	return 1;
 }