arch/arm/boot/compressed/decompress.c: fix build error due to lz4 changes

  include/linux/cpumask.h: In function 'cpumask_parse':
  include/linux/cpumask.h:636:21: error: implicit declaration of function 'strchrnul'; did you mean 'strchr'? [-Werror=implicit-function-declaration]

Because arch/arm/boot/compressed/decompress.c does

#define _LINUX_STRING_H_

preventing linux/string.h from providing strchrnul.  It also #includes
asm/string.h, which for arm has a declaration of strchr(), explaining why
this didn't use to fail.

Link: http://lkml.kernel.org/r/20190528115346.f5a7kn3hdnuf5rts@linutronix.de
Fixes: 3713a4e1 ("include/linux/cpumask.h: fix double string traverse in cpumask_parse")
Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Yury Norov <ynorov@marvell.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm/boot/compressed/decompress.c

@@ -32,6 +32,7 @@
 extern char * strstr(const char * s1, const char *s2);
 extern size_t strlen(const char *s);
 extern int memcmp(const void *cs, const void *ct, size_t count);
+extern char * strchrnul(const char *, int);
 
 #ifdef CONFIG_KERNEL_GZIP
 #include "../../../../lib/decompress_inflate.c"