x86, boot: Fix warning due to undeclared strlen()

Below is a patch that fixes sparse error
"arch/x86/boot/string.c:119:8: warning: symbol 'strlen' was not
declared." by declaring it in arch/x86/boot/boot.h.
Signed-off-by: NFred Chen <fchen@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1376417580-11554-1-git-send-email-fchen@linux.vnet.ibm.comSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/boot.h

@@ -355,6 +355,7 @@ int strncmp(const char *cs, const char *ct, size_t count);
 size_t strnlen(const char *s, size_t maxlen);
 unsigned int atou(const char *s);
 unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int base);
+size_t strlen(const char *s);
 
 /* tty.c */
 void puts(const char *);