updated src/pstdint.h

@desilinguist and I were having trouble compiling the python wrapper on red hat enterprise linux (see https://github.com/kmike/hat-trie/issues/14).  Updating `pstdint.h` to a recent version from what appears to be the "official source" (http://www.azillionmonkeys.com/qed/pstdint.h) seems to fix this.

Oddly enough, we were able to compile the C version of this (separate from the python wrapper) with or without this pull request.  I suppose cython compiles things differently.

Anyway, the `make check` tests pass for me with this change on OS X and red hat.  The python wrapper should also work on both OS X and red hat once this change is pulled in to the wrapper repository as well.