MIPS: ZBOOT: add missing <linux/string.h> include

Commit dc4d7b37 (MIPS: ZBOOT: gather string functions into string.c)
moved the string related functions into a separate file, which might
cause the following build error, depending on the configuration:

| CC      arch/mips/boot/compressed/decompress.o
| In file included from linux/arch/mips/boot/compressed/../../../../lib/decompress_unxz.c:234:0,
|                  from linux/arch/mips/boot/compressed/decompress.c:67:
| linux/arch/mips/boot/compressed/../../../../lib/xz/xz_dec_stream.c: In function 'fill_temp':
| linux/arch/mips/boot/compressed/../../../../lib/xz/xz_dec_stream.c:162:2: error: implicit declaration of function 'memcpy' [-Werror=implicit-function-declaration]
| cc1: some warnings being treated as errors
| linux/scripts/Makefile.build:308: recipe for target 'arch/mips/boot/compressed/decompress.o' failed
| make[6]: *** [arch/mips/boot/compressed/decompress.o] Error 1
| linux/arch/mips/Makefile:308: recipe for target 'vmlinuz' failed

It does not fail with the standard configuration, as when
CONFIG_DYNAMIC_DEBUG is not enabled <linux/string.h> gets included in
include/linux/dynamic_debug.h. There might be other ways for it to
get indirectly included.

We can't add the include directly in xz_dec_stream.c as some
architectures might want to use a different version for the boot/
directory (see for example arch/x86/boot/string.h).

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Cc: stable@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7420/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/boot/compressed/decompress.c

@@ -13,6 +13,7 @@
 
 #include <linux/types.h>
 #include <linux/kernel.h>
+#include <linux/string.h>
 
 #include <asm/addrspace.h>