lib/string_helpers: Add missed declaration of struct task_struct

Starting from the commit 0d0443288f 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: 0d0443288f ("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>

include/linux/string_helpers.h

@@ -5,6 +5,7 @@
 #include <linux/types.h>
 
 struct file;
+struct task_struct;
 
 /* Descriptions of the types of units to
  * print in */