Fix compile issues for docker build

[schroepf]

Fixes a compile issue when building a docker image via

docker build -t vzlogger .

This leads to following error on the apline linux image which is used for the build:

In file included from /vzlogger/src/PushData.cpp:5:
 /vzlogger/include/PushData.hpp:18:27: error: 'int64_t' was not declared in this scope
    18 |         typedef std::pair<int64_t, double> DataTuple;
       |                           ^~~~~~~
 /vzlogger/include/PushData.hpp:14:1: note: 'int64_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
    13 | #include <utility> // for std::pair
   +++ |+#include <cstdint>

[conmarti]

tried the same today, had the same issue and came up with the same solution.
It worked in the past with older versions of gcc (g++)