x86/boot: Add missing declaration of string functions

Add the missing declarations of basic string functions to string.h to allow
a clean build.

Fixes: 5be86566 ("String-handling functions for the new x86 setup code.")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Link: http://lkml.kernel.org/r/1483781911-21399-1-git-send-email-hofrat@osadl.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>