Add copy of opus library in deps/third_party.

opus/silk/float/bwexpander_FLP.c

+/***********************************************************************
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+***********************************************************************/
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "SigProc_FLP.h"
+
+/* Chirp (bw expand) LP AR filter */
+void silk_bwexpander_FLP(
+    silk_float          *ar,                /* I/O  AR filter to be expanded (without leading 1)                */
+    const opus_int      d,                  /* I    length of ar                                                */
+    const silk_float    chirp               /* I    chirp factor (typically in range (0..1) )                   */
+)
+{
+    opus_int   i;
+    silk_float cfac = chirp;
+
+    for( i = 0; i < d - 1; i++ ) {
+        ar[ i ] *=  cfac;
+        cfac    *=  chirp;
+    }
+    ar[ d - 1 ] *=  cfac;
+}