ctrl: Introduce libosmoctrl.map to avoid unintended exports

There are some symbols for use between control_cmd.c and control_if.c,
which are not supposed to be exported publicly.  Let's make sure we
keep those symbols local.

Change-Id: Ia85f36a9c4b2ebf4003718e0a230959638370320
diff --git a/configure.ac b/configure.ac
index 6b9c66f..79ef9d4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -47,11 +47,13 @@
 	LTLDFLAGS_OSMOGB='-Wl,--version-script=$(srcdir)/libosmogb.map'
 	LTLDFLAGS_OSMOGSM='-Wl,--version-script=$(srcdir)/libosmogsm.map'
 	LTLDFLAGS_OSMOCODING='-Wl,--version-script=$(srcdir)/libosmocoding.map'
+	LTLDFLAGS_OSMOCTRL='-Wl,--version-script=$(srcdir)/libosmoctrl.map'
 	;;
 esac
 AC_SUBST(LTLDFLAGS_OSMOGB)
 AC_SUBST(LTLDFLAGS_OSMOGSM)
 AC_SUBST(LTLDFLAGS_OSMOCODING)
+AC_SUBST(LTLDFLAGS_OSMOCTRL)
 
 dnl checks for header files
 AC_HEADER_STDC