externals: Update boost-ext to include safe_numerics.

- This is useful to me for an upcoming change.

externals/boost

@@ -1 +1 @@
-Subproject commit 0b920df1c90f4eb84e2f2b10bee8804b0f6472b2
+Subproject commit 5e8300b76a627f3a1ba215304e04ead33b5bc233