Export __glib_assert_msg

Put __glib_assert_msg in the dynamic symbol table, but not in any public
headers.

This variable is _not_ part of our API but this way debuggers and
automated crash report utilities will be able to access this variable,
even if debug symbols are not available.

https://bugzilla.gnome.org/show_bug.cgi?id=701800

glib/gtestutils.c

@@ -496,6 +496,7 @@
  * analysis systems like Apport and ABRT to fish out assertion messages from
  * core dumps, instead of having to catch them on screen output.
  */
+GLIB_VAR char *__glib_assert_msg;
 char *__glib_assert_msg = NULL;
 
 /* --- constants --- */