kernel.h: Move lib/cmdline.c prototypes to string.h

The lib/cmdline.c is basically a set of some small string parsers
which are wide used in the kernel. Their prototypes belong to the
string.h rather then kernel.h.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20231003130142.2936503-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Kees Cook <keescook@chromium.org>