[Ada] Minor edit to -gnatW message

[Arnaud Charlet]

This patch adds a parameter ? to the -gnatW message for better
consistency with other messages

Compiling:

     1. procedure DummyW is
     2. begin
     3.    null;
     4. end;

with -gnath yields an output file contaning the line:

  -gnatW?   Wide character encoding method (?=h/u/s/e/8/b)

Tested on x86_64-pc-linux-gnu, committed on trunk

2013-04-11  Robert Dewar  <de...@adacore.com>

        * usage.adb (Usage): Minor edit to -gnatW message


Index: usage.adb
===================================================================
--- usage.adb   (revision 197743)
+++ usage.adb   (working copy)
@@ -584,8 +584,8 @@
 
    --  Line for -gnatW switch
 
-   Write_Switch_Char ("W");
-   Write_Str ("Wide character encoding method (");
+   Write_Switch_Char ("W?");
+   Write_Str ("Wide character encoding method (?=");
 
    for J in WC_Encoding_Method loop
       Write_Char (WC_Encoding_Letters (J));