posix: Defined _POSIX_VDISABLE as integer literal

posix: Defined _POSIX_VDISABLE as integer literal

The constant should be used with c_cc, which for all supported ABIs
is defined as unsigned char.  By using it as literar char constant,
clang triggers an error when compared with signal literal on ABIs that
define 'char' as unsigned.

On aarch64, clang shows:

  ../sysdeps/posix/fpathconf.c:118:21: error: right side of operator
  converted from negative value to unsigned: -1 to 18446744073709551615
  [-Werror]
  #if _POSIX_VDISABLE == -1
    ~~~~~~~~~~~~~~~ ^  ~~

Reviewed-by: Collin Funk <collin.funk1@gmail.com>