vt: Move custom isspace() to its own namespace

vt: Move custom isspace() to its own namespace

If by some reason any of the headers will include ctype.h
we will have a name collision. Avoid this by moving isspace()
to the dedicate namespace.

First appearance of the code is in the commit 24a1c2a769cf
("Import 1.1.92").

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210510144947.58270-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>