c++: testsuite: fix cpp0x/initlist135.C portability

The inline definition of std::initializer_list in this testcase is not
what the compiler expects on i686 due to over-reduction of the original
testcase.  Better to just include <initializer_list> instead.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist135.C: Include <initializer_list> and
	remove the inline definition of std::initializer_list.