plug-ins: comment out unused function.

Fix warning:

> plug-ins/file-dds/dxt.c:113:1: warning: unused function 'pack_rgb565'

I don't just remove it as I guess it may be of use at some point for a
not-yet fully implemented feature yet?

plug-ins/file-dds/dxt.c

@@ -108,6 +108,9 @@ extract_block (const unsigned char *src,
     }
 }
 
+#if 0
+/* Currently unused, hidden to avoid compilation warnings. */
+
 /* pack BGR8 to RGB565 */
 static inline unsigned short
 pack_rgb565 (const unsigned char *c)
@@ -116,6 +119,7 @@ pack_rgb565 (const unsigned char *c)
          (mul8bit(c[1], 63) <<  5) |
          (mul8bit(c[0], 31)      );
 }
+#endif
 
 /* unpack RGB565 to BGR */
 static void