mesa: Avoid set comprehension.

Fix build error on CentOS 6.9 with Python 2.6.

  GEN    main/format_fallback.c
  File "./main/format_fallback.py", line 42
    names = {fmt.name for fmt in formats}
                        ^
SyntaxError: invalid syntax

Fixes: a1983223d8 ("mesa: Add _mesa_format_fallback_rgbx_to_rgba() [v2]")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/mesa/main/format_fallback.py

@@ -39,7 +39,7 @@ def parse_args():
     return p.parse_args()
 
 def get_rgbx_to_rgba_map(formats):
-    names = {fmt.name for fmt in formats}
+    names = set(fmt.name for fmt in formats)
 
     for fmt in formats:
         if not fmt.has_channel('r') or not fmt.has_channel('x'):