Add missing index.html file for MPI and GIL.

Ref #2616: All the top level libraries now have index.html files but
several sub-libraries don't. I'm not sure it's worth adding them as many
of them don't have appropriate subdirectories anyway.

[SVN r51387]

libs/gil

@@ -1 +1 @@
-Subproject commit 523fd439fbe403d67ea909d1c6a4e63f01aa0dff
+Subproject commit 9c0e94644e62d6fd0617f27750a50cfcd450fd4a

libs/mpi

@@ -1 +1 @@
-Subproject commit 49100e49cbde273bc2b640f0c602109e226fa6d0
+Subproject commit 3267340308a10b6aba519f6d6a0a4e7107deca65