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: Will Deacon <will.deacon@arm.com>
Cc: David Rusling <david.rusling@linaro.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>