interp: fix receiver for exported method objects

When a function variable is assigned from an exported method,
make sure the receiver is updated from the coresponding symbol.

Fixes #1182.

_test/d1/d1.go

@@ -0,0 +1,9 @@
+package d1
+
+type T struct {
+	Name string
+}
+
+func (t *T) F() { println(t.Name) }
+
+func NewT(s string) *T { return &T{s} }

_test/d2/d2.go

@@ -0,0 +1,8 @@
+package d2
+
+import "github.com/traefik/yaegi/_test/d1"
+
+var (
+	X = d1.NewT("test")
+	F = X.F
+)

_test/d3.go

@@ -0,0 +1,11 @@
+package main
+
+import "github.com/traefik/yaegi/_test/d2"
+
+func main() {
+	f := d2.F
+	f()
+}
+
+// Output:
+// test

interp/cfg.go

@@ -1503,6 +1503,7 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 					n.action = aGetSym
 					n.typ = sym.typ
 					n.sym = sym
+					n.recv = sym.recv
 					n.rval = sym.rval
 				} else {
 					err = n.cfgErrorf("undefined selector: %s.%s", pkg, name)