tool_main.c: fix VMS global variable storage-class specifier

An extern submits a psect and a global reference to the linker to point
to it. Using "extern int vms_show = 0" also creates a globaldef.

The use of the extern by itself does declare a psect but does not declare
a globalsymbol. It does declare a globalref. But the linker needs one and
only one globaldef or there is an error.

src/tool_main.c

@@ -52,7 +52,7 @@
  * Its value may be set in other tool_*.c source files thanks to
  * forward declaration present in tool_vms.h
  */
-static int vms_show = 0;
+extern int vms_show = 0;
 #endif
 
 /*