ARM: decompressor: fix warning introduced in fortify patch

Commit ee333554

 ("ARM: 8749/1: Kconfig: Add ARCH_HAS_FORTIFY_SOURCE")
introduced a new warning:

arch/arm/boot/compressed/misc.c: In function 'fortify_panic':
arch/arm/boot/compressed/misc.c:167:1: error: 'noreturn' function does return [-Werror]

The simple solution would be to make 'error' a noreturn function, but
this causes a prototype mismatch as the function is prototyped in
several .c files.  So, move the function prototype to a new header.

There are also a couple of variables that are also declared in several
locations.  Clean this up while we are here.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>