arm64: kexec: include reboot.h

Include reboot.h in machine_kexec.c for declaration of
machine_crash_shutdown.

gcc-12 with W=1 reports:

 arch/arm64/kernel/machine_kexec.c:257:6: warning: no previous prototype for 'machine_crash_shutdown' [-Wmissing-prototypes]
   257 | void machine_crash_shutdown(struct pt_regs *regs)

No functional changes intended.
Compile tested only.

Signed-off-by: Simon Horman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: horms

arch/arm64/kernel/machine_kexec.c

@@ -11,6 +11,7 @@
 #include <linux/kernel.h>
 #include <linux/kexec.h>
 #include <linux/page-flags.h>
+#include <linux/reboot.h>
 #include <linux/set_memory.h>
 #include <linux/smp.h>