drivers/misc/optee_smc: Fix sched_yield() on flat builds

Previous change that introduced yielding upon OP-TEE
return with reason foreign interrupt, omitted to include
<sched.h>. This worked on kernel builds, but not on flat
builds. Include it.

Signed-off-by: George Poulios <gpoulios@census-labs.com>

drivers/misc/optee_smc.c

@@ -29,6 +29,7 @@
 #include <errno.h>
 #include <syslog.h>
 #include <string.h>
+#include <sched.h>
 
 #include "optee.h"
 #include "optee_smc.h"