import gprs_gsup_message.[ch] from openbsc as gsup.[ch]

Move those routines from OpenBSC to libosmogsm, so they can be
re-used from other programs.  I think it was a mistake to add them only
inside the openbsc repository in the first place.  We need to pay more
attention to this in the future.
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index 4326b35..781f153 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -26,7 +26,8 @@
 			auth_core.c auth_comp128v1.c auth_comp128v23.c \
 			auth_milenage.c milenage/aes-encblock.c \
 			milenage/aes-internal.c milenage/aes-internal-enc.c \
-			milenage/milenage.c gan.c ipa.c gsm0341.c apn.c
+			milenage/milenage.c gan.c ipa.c gsm0341.c apn.c \
+			gsup.c
 libgsmint_la_LDFLAGS = -no-undefined
 libgsmint_la_LIBADD = ../libosmocore.la