xdiff: move stdint.h to xdiff.h
It's more correct to put it in xdiff.h since that file actually uses int64_t
etc and xdiff.h is included by xinclude.h.
This should fix the oss-fuzz build. Thanks durin42 for discovering the
issue.
Test Plan:
`make local` and xdiff related tests still work.
Differential Revision: https://phab.mercurial-scm.org/D2848
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