alpha: boot: fix build breakage introduced by system.h disintegration

Commit ec221208 ("Disintegrate asm/system.h for Alpha") removed the
system.h include from boot/head.S, which puts the PAL_* asm constants
out of scope.

Include <asm/pal.h> so we can get building again.
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Cc: David Rusling <david.rusling@linaro.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/alpha/boot/head.S

@@ -4,6 +4,7 @@
  * initial bootloader stuff..
  */
 
+#include <asm/pal.h>
 
 	.set noreorder
 	.globl	__start