ARM: decompressor: fix warning introduced in fortify patch

Commit ee333554fed5 ("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 <[e-mail]>

arch/arm/boot/compressed/decompress.c

···
 #include <linux/stddef.h>	/* for NULL */
 #include <linux/linkage.h>
 #include <asm/string.h>
-
-extern unsigned long free_mem_ptr;
-extern unsigned long free_mem_end_ptr;
-extern void error(char *);
+#include "misc.h"
 
 #define STATIC static
 #define STATIC_RW_DATA	/* non-static please */

arch/arm/boot/compressed/misc.c

···
 #include <linux/compiler.h>	/* for inline */
 #include <linux/types.h>
 #include <linux/linkage.h>
+#include "misc.h"
 
 static void putstr(const char *ptr);
-extern void error(char *x);
 
 #include CONFIG_UNCOMPRESS_INCLUDE
 

arch/arm/boot/compressed/misc.h

···
+#ifndef MISC_H
+#define MISC_H
+
+#include <linux/compiler.h>
+
+void error(char *x) __noreturn;
+extern unsigned long free_mem_ptr;
+extern unsigned long free_mem_end_ptr;
+
+#endif