libstdc++: Add missing constraints to views::indices

libstdc++: Add missing constraints to views::indices

Calling views::indices(n) should be expression equivalent to
views::iota(decltype(n)(0), n), which means it should have the same
constraints as views::iota and be SFINAE-friendly.

libstdc++-v3/ChangeLog:

	* include/std/ranges (indices::operator()): Constrain using
	__can_iota_view concept.
	* testsuite/std/ranges/indices/1.cc: Check SFINAE-friendliness
	required by expression equivalence. Replace unused <vector>
	header with <stddef.h> needed for size_t.