doxy: Define a group for libswscale documentation

Uniform the style and eliminate a warning on newer doxygen version..

libavutil/avutil.h

@@ -40,7 +40,7 @@
  * @li @ref lavd "libavdevice" special devices muxing/demuxing library
  * @li @ref lavu "libavutil" common utility library
  * @li @ref lavr "libavresample" audio resampling, format conversion and mixing
- * @li @subpage libswscale  color conversion and scaling library
+ * @li @ref libsws "libswscale"  color conversion and scaling library
  *
  * @section libav_versioning Versioning and compatibility
  *

libswscale/swscale.h

@@ -23,6 +23,7 @@
 
 /**
  * @file
+ * @ingroup libsws
  * @brief
  *     external api for the swscale stuff
  */
@@ -35,6 +36,9 @@
 #include "version.h"
 
 /**
+ * @defgroup libsws Color conversion and scaling
+ * @{
+ *
  * Return the LIBSWSCALE_VERSION_INT constant.
  */
 unsigned swscale_version(void);
@@ -343,4 +347,8 @@ void sws_convertPalette8ToPacked24(const uint8_t *src, uint8_t *dst, int num_pix
  */
 const AVClass *sws_get_class(void);
 
+/**
+ * @}
+ */
+
 #endif /* SWSCALE_SWSCALE_H */