x86/boot: Include missing header file

Sparse complains about missing forward declarations:

arch/x86/boot/compressed/error.c:8:6:
	warning: symbol 'warn' was not declared. Should it be static?
arch/x86/boot/compressed/error.c:15:6:
	warning: symbol 'error' was not declared. Should it be static?

Include the missing header file.
Signed-off-by: Zhengyi Shen <shenzhengyi@gmail.com>
Acked-by: Kess Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/1490770820-24472-1-git-send-email-shenzhengyi@gmail.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/boot/compressed/error.c

@@ -4,6 +4,7 @@
  * memcpy() and memmove() are defined for the compressed boot environment.
  */
 #include "misc.h"
+#include "error.h"
 
 void warn(char *m)
 {