ARM: 8654/1: decompressor: add strlen prototype

The decompress.c file contains a declaration for strstr() so we can
include some compression library code.

With the updated LZ4 implementation, we run into the same problem again
for strlen():

In file included from ../include/linux/rcupdate.h:40:0,
                 from ../include/linux/srcu.h:33,
                 from ../include/linux/notifier.h:15,
                 from ../include/linux/memory_hotplug.h:6,
                 from ../include/linux/mmzone.h:749,
                 from ../include/linux/gfp.h:5,
                 from ../include/linux/kmod.h:22,
                 from ../include/linux/module.h:13,
                 from ../arch/arm/boot/compressed/../../../../lib/lz4/lz4_decompress.c:39,
                 from ../arch/arm/boot/compressed/../../../../lib/decompress_unlz4.c:13,
                 from ../arch/arm/boot/compressed/decompress.c:55:
include/linux/cpumask.h: In function 'cpumask_parse':
include/linux/cpumask.h:592:53: error: implicit declaration of function 'strlen';did you mean 'strstr'? [-Werror=implicit-function-declaration]

This adds another declaration to work around the new problem.

Fixes: ce83d9ab80d6 ("lib: update LZ4 compressor module")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>