Alpha: move dsputil prototypes to a header file

Originally committed as revision 22308 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/alpha/simple_idct_alpha.c

@@ -27,13 +27,9 @@
  */
 
 #include "libavcodec/dsputil.h"
+#include "dsputil_alpha.h"
 #include "asm.h"
 
-extern void (*put_pixels_clamped_axp_p)(const DCTELEM *block, uint8_t *pixels,
-                                        int line_size);
-extern void (*add_pixels_clamped_axp_p)(const DCTELEM *block, uint8_t *pixels,
-                                        int line_size);
-
 // cos(i * M_PI / 16) * sqrt(2) * (1 << 14)
 // W4 is actually exactly 16384, but using 16383 works around
 // accumulating rounding errors for some encoders