girepository: Remove _ prefix from private functions

Now that libgirepository uses `GI_AVAILABLE_IN_*` macros, that’s what
controls symbol visibility. The `_` prefixes are redundant, and out of
keeping with the rest of GLib.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3155

girepository/gifunctioninfo.c

@@ -48,10 +48,10 @@
  */
 
 GIFunctionInfo *
-_gi_base_info_find_method (GIBaseInfo  *base,
-                           guint32      offset,
-                           gint         n_methods,
-                           const gchar *name)
+gi_base_info_find_method (GIBaseInfo  *base,
+                          guint32      offset,
+                          gint         n_methods,
+                          const gchar *name)
 {
   /* FIXME hash */
   GIRealInfo *rinfo = (GIRealInfo*)base;