Fix building on AIX 5.1

[likema]

When building zstd by GCC 4.5.4 on AIX 5.1 , GCC could not found stdint.h

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[Cyan4973]

I'm fine with the proposed update,
however, it happens there is also an ongoing patch
which changes the way zstd includes C standard libraries and defines internal types,
and it will conflict with this PR.
cc @terrelln

[Cyan4973]

With mem.h now updated,
could you please rebase your PR onto latest dev branch ?
Once conflict is cleared, I believe it's good to go.

[likema]

I have rebased it onto dev.

But I find that

• contrib/pzstd/test/RoundTrip.h
• contrib/pzstd/utils/FileSystem.h
• contrib/pzstd/Options.h
• contrib/pzstd/SkippableFrame.h
• contrib/pzstd/Pzstd.h
• contrib/single_file_libs/examples/roundtrip.c
• contrib/single_file_libs/examples/simple.c
• doc/educational_decoder/zstd_decompress.c

includes cstdint

Should I fix them?

[Cyan4973]

Nope, these are different programs, just hosted in the same repository.
The current scope of this PR looks good to me.