arm64: ssbd: explicitly depend on <linux/prctl.h>

Fix ssbd.c which depends implicitly on asm/ptrace.h including
linux/prctl.h (through for example linux/compat.h, then linux/time.h,
linux/seqlock.h, linux/spinlock.h and linux/irqflags.h), and uses
PR_SPEC* defines.

This is an issue since we'll soon be removing the include from
asm/ptrace.h.

Fixes: 9cdc0108 ("arm64: ssbd: Add prctl interface for per-thread mitigation")
Cc: stable@vger.kernel.org
Signed-off-by: NAnisse Astier <aastier@freebox.fr>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/ssbd.c

@@ -5,6 +5,7 @@
 
 #include <linux/compat.h>
 #include <linux/errno.h>
+#include <linux/prctl.h>
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
 #include <linux/thread_info.h>