add missing include: cstdint

[zeuner]

Depending on the build platform, ItemType and DirItemType without including cstdint first leads to compile errors:

In file included from /build/source/Basics/Basics.h:5,
                 from /build/source/Trees/Trees.h:4,
                 from /build/source/Pool/Pool.h:7,
                 from /build/source/Pool/Functions.h:11,
                 from /build/source/Pool/Functions.cpp:8:
/build/source/Basics/Enums.h:13:11: warning: elaborated-type-specifier for a scoped enum must not use the 'class' keyword
   13 |     enum  class ItemType : uint8_t {
      |     ~~~~  ^~~~~
      |           -----
/build/source/Basics/Enums.h:13:26: error: found ':' in nested-name-specifier, expected '::'
   13 |     enum  class ItemType : uint8_t {

[...]

(fixed by this PR)