Make ctype_.c and ctype_.h agree on _ctype_b type

_ctype_b is defined in ctype_.c as a const char array for non cygwin
targets allowing negative ctype index but as a char array for the same
targets in ctype_.h, giving type conflict at compile time.  This is
because the cygwin targets are not treated specially in the latter file.
This patch adds the necessary logic for cygwin targets in ctype_.h.

newlib/libc/ctype/ctype_.h

@@ -6,7 +6,11 @@
 
 #ifdef ALLOW_NEGATIVE_CTYPE_INDEX
 
+#ifndef __CYGWIN__
+  extern _CONST char _ctype_b[];
+#else
   extern char _ctype_b[];
+#endif
 # define DEFAULT_CTYPE_PTR	((char *) _ctype_b + 127)
 
 #else	/* !ALLOW_NEGATIVE_CTYPE_INDEX */