Fix counter of fixed keyframe distance

When the keyframe distance is fixed the first interval has the right
distance but, the next ones have kf_distance + 1.

Change-Id: I44f1190fe7146124bd07660a5e0ef08829e3ae07

vp8/vp8_cx_iface.c

@@ -706,7 +706,7 @@ static vpx_codec_err_t vp8e_encode(vpx_codec_alg_priv_t  *ctx,
         if (++ctx->fixed_kf_cntr > ctx->cfg.kf_min_dist)
         {
             flags |= VPX_EFLAG_FORCE_KF;
-            ctx->fixed_kf_cntr = 0;
+            ctx->fixed_kf_cntr = 1;
         }
     }