Enable 'c-c++-common/goacc/{if,self}-clause-1.c' for C++

Enable 'c-c++-common/goacc/{if,self}-clause-1.c' for C++

As discovered via recent
commit 3a3596389c2e539cb8fd5dc5784a4e2afe193a2a
"OpenACC 2.7: Implement self clause for compute constructs",
'c-c++-common/goacc/if-clause-1.c', which the new
'c-c++-common/goacc/self-clause-1.c' was copied from, was not enabled for C++.

	gcc/testsuite/
	* c-c++-common/goacc/if-clause-1.c: Enable for C++
	* c-c++-common/goacc/self-clause-1.c: Likewise.