config: move struct to where it belongs

it is used only to define the structure of `categories_array` which
lives in `vlc_config_cat.h`; it belongs next to that.

include/vlc_config_cat.h

@@ -192,6 +192,13 @@
 #define ANETWORK_TITLE N_( "Network" )
 #define ANETWORK_HELP N_( "Advanced network settings." )
 
+struct config_category_t
+{
+    int         i_id;
+    const char *psz_name;
+    const char *psz_help;
+};
+
 static const struct config_category_t categories_array[] =
 {
     /* Interface */

include/vlc_configuration.h

@@ -49,13 +49,6 @@
 extern "C" {
 # endif
 
-struct config_category_t
-{
-    int         i_id;
-    const char *psz_name;
-    const char *psz_help;
-};
-
 typedef union
 {
     char       *psz;