x86/boot: Add missing strchr() declaration

The Sparse static analyzer emits this warning:

    symbol 'strchr' was not declared. Should it be static?

This patch adds the appropriate extern declaration to string.h
to fix the warning.

Signed-off-by: Tommy Nguyen <remyabel@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170623143601.GA20743@NoChina


Signed-off-by: Ingo Molnar <mingo@kernel.org>