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

arch/x86/boot/string.h

@@ -22,6 +22,7 @@ extern int strcmp(const char *str1, const char *str2);
 extern int strncmp(const char *cs, const char *ct, size_t count);
 extern size_t strlen(const char *s);
 extern char *strstr(const char *s1, const char *s2);
+extern char *strchr(const char *s, int c);
 extern size_t strnlen(const char *s, size_t maxlen);
 extern unsigned int atou(const char *s);
 extern unsigned long long simple_strtoull(const char *cp, char **endp,