x86: add necessary includes for prom.h

Once prom.h is no longer implicitly included, we need to include setup.h
to get COMMAND_LINE_SIZE.

Signed-off-by: Rob Herring <[e-mail]>
Acked-by: Grant Likely <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: [e-mail]

arch/x86/kernel/devicetree.c

···
 #include <asm/hpet.h>
 #include <asm/apic.h>
 #include <asm/pci_x86.h>
+#include <asm/setup.h>
 
 __initdata u64 initial_dtb;
 char __initdata cmd_line[COMMAND_LINE_SIZE];