testsuite: Require LTO support for pr94426-1.C

The testcase uses the -flto option but does not ensure that LTO support
is enabled. This patch adds the test to the testcase.

        * g++.dg/cpp0x/lambda/pr94426-1.C: Require LTO.

gcc/testsuite/ChangeLog

+2020-04-23  David Edelsohn  <dje.gcc@gmail.com>
+
+	* g++.dg/cpp0x/lambda/pr94426-1.C: Require LTO.
+
 2020-04-23  Martin Sebor  <msebor@redhat.com>
 
 	PR driver/90983

gcc/testsuite/g++.dg/cpp0x/lambda/pr94426-1.C

 // { dg-do compile { target c++14 } }
 // PR 94426 ICE mangling lambda
 // { dg-options {-flto -O2} }
+// { dg-require-effective-target lto }
 
 template <bool> using Void = void;