gsm0808: Add utils for AoIP Transport Layer Address

The planned support for true A over IP requires the encoding and
decoding of a so called "AoIP Transport Layer Address" element.

This commt adds parsing functionality and tests for the element
mentioned above, however, it is not yet actively used.

Change-Id: I57933b0a06a3f54ec2a41e6ecb6ced9fbbc89332
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index 3a4a0cd..e64c9e7 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -25,7 +25,7 @@
 			auth_milenage.c milenage/aes-encblock.c gea.c \
 			milenage/aes-internal.c milenage/aes-internal-enc.c \
 			milenage/milenage.c gan.c ipa.c gsm0341.c apn.c \
-			gsup.c gprs_gea.c gsm0503_conv.c oap.c
+			gsup.c gprs_gea.c gsm0503_conv.c oap.c gsm0808_utils.c
 libgsmint_la_LDFLAGS = -no-undefined
 libgsmint_la_LIBADD = $(top_builddir)/src/libosmocore.la