plug-ins: fix #5313 Exporting to "Gimp pattern" i.e. pat format fails

After the change that allows multiple layers to be selected exporting to
a pattern fails.

Patch this as suggested by Lloyd Konneker by doing the same as for brushes:
do not allow multiple layers, instead only send the first drawable.

plug-ins/common/file-pat.c

@@ -229,8 +229,7 @@ pat_save (GimpProcedure        *procedure,
                                 "file-pat-save-internal",
                                 GIMP_TYPE_RUN_MODE,     GIMP_RUN_NONINTERACTIVE,
                                 GIMP_TYPE_IMAGE,        image,
-                                G_TYPE_INT,             n_drawables,
-                                GIMP_TYPE_OBJECT_ARRAY, drawables,
+                                GIMP_TYPE_DRAWABLE,     drawables[0],
                                 G_TYPE_FILE,            file,
                                 G_TYPE_STRING,          description,
                                 G_TYPE_NONE);