Add some missing 'const' in version.c's string data.

I can't believe this codebase is around 20 years old and has had
multiple giant const-fixing patches, and yet there are _still_ things
that should have been const for years and aren't.

putty.h

@@ -1158,7 +1158,7 @@ void ser_setup_config_box(struct controlbox *b, int midsession,
 /*
  * Exports from version.c.
  */
-extern char ver[];
+extern const char ver[];
 
 /*
  * Exports from unicode.c.

ssh.h

@@ -489,7 +489,7 @@ void aes_ssh2_decrypt_blk(void *handle, unsigned char *blk, int len);
 /*
  * PuTTY version number formatted as an SSH version string. 
  */
-extern char sshver[];
+extern const char sshver[];
 
 /*
  * Gross hack: pscp will try to start SFTP but fall back to scp1 if

version.c

@@ -10,8 +10,8 @@
 
 #include "version.h"
 
-char ver[] = TEXTVER;
-char sshver[] = SSHVER;
+const char ver[] = TEXTVER;
+const char sshver[] = SSHVER;
 
 /*
  * SSH local version string MUST be under 40 characters. Here's a

windows/winpgen.c

@@ -134,7 +134,7 @@ static void progress_update(void *param, int action, int phase, int iprogress)
     }
 }
 
-extern char ver[];
+extern const char ver[];
 
 struct PassphraseProcStruct {
     char **passphrase;

windows/winpgnt.c

@@ -49,7 +49,7 @@
 
 #define APPNAME "Pageant"
 
-extern char ver[];
+extern const char ver[];
 
 static HWND keylist;
 static HWND aboutbox;