rest_octets: Actually include gsm48_rest_octets.c in Makefile.am

The gsm48_rest_octets.c file was added in Change-Id
I47888965ab11bba1186c21987f1365c9270abeab, but it never actually
ended up being compiled as it wasn't listed in the makefile :/

Change-Id: I0355115c2645f236c9e32cf7a563cf51a857e8a3
Relsted: OS#3075
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index 658ef2d..5740b67 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -32,7 +32,7 @@
 			milenage/milenage.c gan.c ipa.c gsm0341.c apn.c \
 			gsup.c gsup_sms.c gprs_gea.c gsm0503_conv.c oap.c gsm0808_utils.c \
 			gsm23003.c mncc.c bts_features.c oap_client.c \
-			gsm29118.c
+			gsm29118.c gsm48_rest_octets.c
 libgsmint_la_LDFLAGS = -no-undefined
 libgsmint_la_LIBADD = $(top_builddir)/src/libosmocore.la