Do not drop const qualifier for Basename

Do not drop const qualifier for Basename

The private Basename() implementation does not modify its argument, so
a cast to a non-const char pointer is not necessary.

newgrp.c:790:39: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
  790 |         progbase = (char *) Basename ((char *) prog);
      |                                       ^
newgrp.c:790:20: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
  790 |         progbase = (char *) Basename ((char *) prog);
      |                    ^

shell.c:48:70: warning: cast discards ‘const’ qualifier from pointer target type [-Wcast-qual]
   48 |                 (void) snprintf (arg0, sizeof arg0, "-%s", Basename ((char *) file));
      |                                                                      ^