[OCaml] Bump ctypes dependency to 0.4.

[OCaml] Bump ctypes dependency to 0.4.

ctypes 0.3 and earlier contains an interface-definig bug:
its ptr_of_raw_address accepts Int64 and not Nativeint. ctypes 0.4
was not released during the 3.6 cycle, and because of that, LLVM 3.6
was released with ctypes 0.3 as a dependency, which now breaks
the build on modern ctypes.

Unbreak.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240882 91177308-0d34-0410-b5e6-96231b3b80d8