glsl2: Update the callee pointer of calls to newly-linked-in functions.

Otherwise, ir_function_inlining will see the body of the function from
the unlinked version of the shader, which won't have had the lowering
passes done on it or linking's variable remapping.

src/glsl/link_functions.cpp

@@ -164,6 +164,8 @@ public:
        */
       linked_sig->accept(this);
 
+      ir->set_callee(linked_sig);
+
       return visit_continue;
    }