cosmetic: codec/Makefile.am: list sources one file per line

Change-Id: I299fdb887f640151bf57d6423b0e08ffa8cd02f8
diff --git a/src/codec/Makefile.am b/src/codec/Makefile.am
index f7877fe..d3718c3 100644
--- a/src/codec/Makefile.am
+++ b/src/codec/Makefile.am
@@ -13,7 +13,14 @@
 
 lib_LTLIBRARIES = libosmocodec.la
 
-libosmocodec_la_SOURCES = gsm610.c gsm620.c gsm660.c gsm690.c ecu.c ecu_fr.c \
-	ecu_fr_old.c
+libosmocodec_la_SOURCES = \
+	gsm610.c \
+	gsm620.c \
+	gsm660.c \
+	gsm690.c \
+	ecu.c \
+	ecu_fr.c \
+	ecu_fr_old.c \
+	$(NULL)
 libosmocodec_la_LDFLAGS = -version-info $(LIBVERSION) -no-undefined
 libosmocodec_la_LIBADD = $(top_builddir)/src/core/libosmocore.la