arm: vp3: remove incorrect const in ff_vp3_idct_dc_add_neon declaration

Was missed in aeaf268e52 when integrating
clear_blocks into the idct.

libavcodec/arm/vp3dsp_init_arm.c

@@ -26,7 +26,7 @@
 
 void ff_vp3_idct_put_neon(uint8_t *dest, int line_size, int16_t *data);
 void ff_vp3_idct_add_neon(uint8_t *dest, int line_size, int16_t *data);
-void ff_vp3_idct_dc_add_neon(uint8_t *dest, int line_size, const int16_t *data);
+void ff_vp3_idct_dc_add_neon(uint8_t *dest, int line_size, int16_t *data);
 
 void ff_vp3_v_loop_filter_neon(uint8_t *, int, int *);
 void ff_vp3_h_loop_filter_neon(uint8_t *, int, int *);