arm: asm decode_block_coeffs_internal is vp8 specific

Unbreaks compilation on arm due to conflicting types for
'ff_decode_block_coeffs_armv6'.

libavcodec/arm/vp8.h

@@ -26,7 +26,7 @@
 #include "libavcodec/vp8.h"
 
 #if HAVE_ARMV6_EXTERNAL
-#define decode_block_coeffs_internal ff_decode_block_coeffs_armv6
+#define vp8_decode_block_coeffs_internal ff_decode_block_coeffs_armv6
 int ff_decode_block_coeffs_armv6(VP56RangeCoder *rc, int16_t block[16],
                                  uint8_t probs[8][3][NUM_DCT_TOKENS-1],
                                  int i, uint8_t *token_prob, int16_t qmul[2]);