Fix Nanos6 4.0 build

It looks like after upgrading the compiler the build breaks. The patch
simply adds the missing cstdint include, until a new release is made.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Tested-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>