shape.c: make internal_object_id extern instead of static

Otherwise every inclusion of shape.h result in a different variable
and only the one in `shape.c` is properly initialized.

Author: byroot

shape.c

@@ -47,6 +47,7 @@
 
 static ID id_frozen;
 static ID id_t_object;
+ID internal_object_id; // extern
 
 #define LEAF 0
 #define BLACK 0x0

shape.h

@@ -39,7 +39,7 @@ typedef uint32_t redblack_id_t;
 //      ROOT_TOO_COMPLEX_SHAPE_ID   0x2
 #define FIRST_T_OBJECT_SHAPE_ID     0x3
 
-static ID internal_object_id;
+extern ID internal_object_id;
 
 typedef struct redblack_node redblack_node_t;