lavc/wma: add av_warn_unused_result to ff_wma_init

May help to prevent incidents like 19e456d4.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/wma.h

@@ -144,7 +144,9 @@ extern const float ff_wma_lsp_codebook[NB_LSP_COEFS][16];
 extern const uint32_t ff_aac_scalefactor_code[121];
 extern const uint8_t  ff_aac_scalefactor_bits[121];
 
+av_warn_unused_result
 int ff_wma_init(AVCodecContext *avctx, int flags2);
+
 int ff_wma_total_gain_to_bits(int total_gain);
 int ff_wma_end(AVCodecContext *avctx);
 unsigned int ff_wma_get_large_val(GetBitContext *gb);