lib/string_helpers: Add missed declaration of struct task_struct

Starting from the commit 0d044328 the new function has been
introduced which takes struct task_struct as a parameter. Though,
compiler doesn't know where to get information about it at this stage.

Add missed declaration of struct task_struct to satisfy compiler.

Fixes: 0d044328 ("string_helpers: add kstrdup_quotable_cmdline")
Cc: Kees Cook <keescook@chromium.org>
Cc: James Morris <james.l.morris@oracle.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Kees Cook <keescook@chromium.org>