Add img_convert() to imgconvert.h and #include it from imgresample.c.

This fixes an implicit declaration of img_convert() warning.
taken from a patchset from Anders Grönberg, galileo.m2 gmail com

Originally committed as revision 16097 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/imgconvert.h

@@ -33,4 +33,7 @@ int ff_fill_pointer(AVPicture *picture, uint8_t *ptr, int pix_fmt, int height);
 
 int ff_get_plane_bytewidth(enum PixelFormat pix_fmt, int width, int plane);
 
+int img_convert(AVPicture *dst, int dst_pix_fmt, const AVPicture *src,
+                int src_pix_fmt, int src_width, int src_height);
+
 #endif /* AVCODEC_IMGCONVERT_H */

libavcodec/imgresample.c

@@ -26,6 +26,7 @@
 
 #include "avcodec.h"
 #include "dsputil.h"
+#include "imgconvert.h"
 #include "libswscale/swscale.h"
 
 #ifdef HAVE_ALTIVEC