x86/KASLR: Use the right memcpy() implementation

The decompressor has its own implementation of the string functions,
but has to include the right header to get those, while implicitly
including linux/string.h may result in a link error:

  arch/x86/boot/compressed/kaslr.o: In function `choose_random_location':
  kaslr.c:(.text+0xf51): undefined reference to `_mmx_memcpy'

This has appeared now as KASLR started using memcpy(), via:

	d52e7d5a ("x86/KASLR: Parse all 'memmap=' boot option entries")

Other files in the decompressor already do the same thing.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170530091446.1000183-1-arnd@arndb.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/kaslr.c

@@ -30,6 +30,7 @@
 
 #include "misc.h"
 #include "error.h"
+#include "../string.h"
 
 #include <generated/compile.h>
 #include <linux/module.h>