Avoid testcase warning on arm ABI [pr94608]

	PR c++/94608
	* g++.dg/lto/pr83720_0.C: Add fn def to avoid warning on arm ABI.

gcc/testsuite/ChangeLog

+2020-04-17  Nathan Sidwell  <nathan@acm.org>
+
+	PR c++/94608
+	* g++.dg/lto/pr83720_0.C: Add fn def to avoid warning on arm ABI.
+
 2020-04-17  Jakub Jelinek  <jakub@redhat.com>
 
 	PR rtl-optimization/94618

gcc/testsuite/g++.dg/lto/pr83720_0.C

@@ -48,7 +48,7 @@ public:
   k(al<void(b::ai)>);
 } d([](b::ai) {
   struct be {
-    virtual void f();
+    virtual void f(){}
   };
   struct bf;
   b::c<bf>().aj("", &be::f);