In the mips CONFIG_SYS_SUPPORTS_ZBOOT kernel, fix the compile error
when using CONFIG_FORTIFY_SOURCE=y

LD      vmlinuz
mipsel-openwrt-linux-musl-ld: arch/mips/boot/compressed/decompress.o: in
function `decompress_kernel':
./include/linux/decompress/mm.h:(.text.decompress_kernel+0x177c):
undefined reference to `warn_slowpath_fmt'

kernel test robot helped identify this as related to fortify. The error
appeared with commit 54d9469b ("fortify: Add run-time WARN for
cross-field memcpy()")
Link: https://lore.kernel.org/r/202209161144.x9xSqNQZ-lkp@intel.com/

Resolve this in the same style as commit cfecea6e ("lib/string:
Move helper functions out of string.c")
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 54d9469b ("fortify: Add run-time WARN for cross-field memcpy()")
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>