libosmocodec link fix on MacOSX

Fixes the following bug:

  CCLD     libosmocodec.la
Undefined symbols for architecture x86_64:
  "_bitvec_get_bit_pos", referenced from:
      _osmo_fr_check_sid in gsm610.o
  "_bitvec_get_uint", referenced from:
      _osmo_hr_check_sid in gsm620.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [libosmocodec.la] Error 1

Change-Id: Id7358b94e274b529a8da38a0b2ef8c892b6dd7a5
diff --git a/src/codec/Makefile.am b/src/codec/Makefile.am
index 619897f..5c69c28 100644
--- a/src/codec/Makefile.am
+++ b/src/codec/Makefile.am
@@ -9,3 +9,4 @@
 
 libosmocodec_la_SOURCES = gsm610.c gsm620.c gsm660.c gsm690.c
 libosmocodec_la_LDFLAGS = -version-info $(LIBVERSION) -no-undefined
+libosmocodec_la_LIBADD = $(top_builddir)/src/libosmocore.la