libosmocodec: implement ECU (Error Concealment Unit) for FR

When a bad GSM voice frame is received, it's being replaced
by a silence frame. This may cause unpleasant audio effects.

This change implements a functionality to craft a replacement
frame from the last known good frame. Currently, only FR is
supported, support for other codecs may be added latter.

Change-Id: I06a21f60db01bfe1c2b838f93866fad1d53fdcd1
diff --git a/src/codec/Makefile.am b/src/codec/Makefile.am
index b4df5da..f77d910 100644
--- a/src/codec/Makefile.am
+++ b/src/codec/Makefile.am
@@ -13,6 +13,6 @@
 
 lib_LTLIBRARIES = libosmocodec.la
 
-libosmocodec_la_SOURCES = gsm610.c gsm620.c gsm660.c gsm690.c
+libosmocodec_la_SOURCES = gsm610.c gsm620.c gsm660.c gsm690.c ecu_fr.c
 libosmocodec_la_LDFLAGS = -version-info $(LIBVERSION) -no-undefined
 libosmocodec_la_LIBADD = $(top_builddir)/src/libosmocore.la