Expose str::SplitInclusive in alloc and therefore in std

This seems to have been omitted from the beginning when this feature
was first introduced in 86bf9629.

Most users won't need to name this type which is probably why this
wasn't noticed in the meantime.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

library/alloc/src/str.rs

@@ -46,6 +46,8 @@
 pub use core::str::EncodeUtf16;
 #[stable(feature = "split_ascii_whitespace", since = "1.34.0")]
 pub use core::str::SplitAsciiWhitespace;
+#[stable(feature = "split_inclusive", since = "1.51.0")]
+pub use core::str::SplitInclusive;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::str::SplitWhitespace;
 #[stable(feature = "rust1", since = "1.0.0")]