Makefile.am: fix missing LTLDFLAGS for libosmocoding

Despite the libosmocoding.map is preset since the library release,
one was not used in a proper way. The LTLDFLAGS were missing, so
let's add them.

Change-Id: Idf677825ff642d50bea43c7f970810783e864fdd
diff --git a/configure.ac b/configure.ac
index 5821ad3..33c151e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -51,6 +51,7 @@
 esac
 AC_SUBST(LTLDFLAGS_OSMOGB)
 AC_SUBST(LTLDFLAGS_OSMOGSM)
+AC_SUBST(LTLDFLAGS_OSMOCODING)
 
 dnl checks for header files
 AC_HEADER_STDC