editorconfig: Fix up the tab rendering width.

Our editorconfig file looked sensible, saying that we wanted to indent
with spaces and use 3/4/whatever space indentation.  However, the spec has
this little surprise:

    "tab_width: a whole number defining the number of columns used to
     represent a tab character. This defaults to the value of indent_size
     and doesn't usually need to be specified."

so once my editor started respecting editorconfig, the files that have
tabs left in them started getting rendered wrong, showing up like this in
brw_program.c:

   case GL_COMPUTE_PROGRAM_NV: {
      struct brw_program *prog = rzalloc(NULL, struct brw_program);
      if (prog) {
    prog->id = get_new_program_id(brw->screen);

    return _mesa_init_gl_program(&prog->program, target, id);
      }
      else
    return NULL;
   }

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

.editorconfig

@@ -6,6 +6,7 @@ root = true
 [*]
 charset = utf-8
 insert_final_newline = true
+tab_width = 8
 
 [*.{c,h,cpp,hpp,cc,hh}]
 indent_style = space