openbsc: add missing AC_LANG_SOURCE for current autoconf

autoconf >=2.68 demands the body of an AC_COMPILE_IFELSE to be wrapped in
an AC_LANG_SOURCE macro, otherwise outputs annoying warnings on the
invocation of `autoreconf -i`. this patch follows that demand.

openbsc/configure.in

@@ -59,7 +59,7 @@ dnl Checks for typedefs, structures and compiler characteristics
 saved_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -fvisibility=hidden "
 AC_MSG_CHECKING([if ${CC} supports -fvisibility=hidden])
-AC_COMPILE_IFELSE([char foo;],
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([char foo;])],
       [ AC_MSG_RESULT([yes])
         SYMBOL_VISIBILITY="-fvisibility=hidden"],
         AC_MSG_RESULT([no]))