src/StringFunctions.h: add missing <cstdint> header

[trofi]

Without the change build fails on weekly gcc-13 as:

In file included from src/StructureNode.cpp:32:
src/StringFunctions.h:56:34:
  error: 'uint8_t' was not declared in this scope
   56 |    inline std::string hexString( uint8_t x )
      |                                  ^~~~~~~
src/StringFunctions.h:32:1:
  note: 'uint8_t' is defined in header '<cstdint>';
    did you forget to '#include <cstdint>'?
   31 | #include <iostream>
  +++ |+#include <cstdint>
   32 | #include <sstream>

[asmaloney]

Thanks Sergei!