file.c: ALT_SEPARATOR on cygwin

* file.c (file_alt_separator): define only on DOSISH platforms.
  File::ALT_SEPARATOR is nil on cygwin.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60159 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

file.c

@@ -2991,7 +2991,9 @@ rb_file_s_umask(int argc, VALUE *argv)
 #endif
 #ifdef FILE_ALT_SEPARATOR
 #define isdirsep(x) ((x) == '/' || (x) == FILE_ALT_SEPARATOR)
+# ifdef DOSISH
 static const char file_alt_separator[] = {FILE_ALT_SEPARATOR, '\0'};
+# endif
 #else
 #define isdirsep(x) ((x) == '/')
 #endif