x86/boot: Declare error() as noreturn

The compressed boot function error() is used to halt execution, but it
wasn't marked with "noreturn". This fixes that in preparation for
supporting kernel FORTIFY_SOURCE, which uses the noreturn annotation
on panic, and calls error(). GCC would warn about a noreturn function
calling a non-noreturn function:

  arch/x86/boot/compressed/misc.c: In function ‘fortify_panic’:
  arch/x86/boot/compressed/misc.c:416:1: warning: ‘noreturn’ function does return
   }
 ^
Signed-off-by: NKees Cook <keescook@chromium.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/20170506045116.GA2879@beastSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/error.h

 #ifndef BOOT_COMPRESSED_ERROR_H
 #define BOOT_COMPRESSED_ERROR_H
 
+#include <linux/compiler.h>
+
 void warn(char *m);
-void error(char *m);
+void error(char *m) __noreturn;
 
 #endif /* BOOT_COMPRESSED_ERROR_H */