Rename preemphasis() to celt_preemphasis() to avoid amrnb symbol clash

http://lists.xiph.org/pipermail/opus/2013-November/002372.html
diff --git a/celt/celt_encoder.c b/celt/celt_encoder.c
index 784d432..ba78ea9 100644
--- a/celt/celt_encoder.c
+++ b/celt/celt_encoder.c
@@ -450,7 +450,7 @@
 }
 
 
-void preemphasis(const opus_val16 * OPUS_RESTRICT pcmp, celt_sig * OPUS_RESTRICT inp,
+void celt_preemphasis(const opus_val16 * OPUS_RESTRICT pcmp, celt_sig * OPUS_RESTRICT inp,
                         int N, int CC, int upsample, const opus_val16 *coef, celt_sig *mem, int clip)
 {
    int i;
@@ -1452,7 +1452,7 @@
       enc->nbits_total+=tell-ec_tell(enc);
    }
    c=0; do {
-      preemphasis(pcm+c, in+c*(N+st->overlap)+st->overlap, N, CC, st->upsample,
+      celt_preemphasis(pcm+c, in+c*(N+st->overlap)+st->overlap, N, CC, st->upsample,
                   mode->preemph, st->preemph_memE+c, st->clip);
    } while (++c<CC);