libstdc++: Remove incorrect static specifiers

These functions were originally static members of the path class, but
the 'static' specifiers were not removed when they were moved to
namespace scope. This causes ODR violations when the functions are
called from functions defined in the header, which is incompatible with
Nathan's modules branch.  Change them to 'inline' instead.

	* include/bits/fs_path.h (__detail::_S_range_begin)
	(__detail::_S_range_end): Remove unintentional static specifiers.
	* include/experimental/bits/fs_path.h (__detail::_S_range_begin)
	(__detail::_S_range_end): Likewise.