Include <stdint.h> for uint64_t.

[BillyONeal]

Resolves build errors on Ubuntu 24.04:

In file included from /vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/issuing-authority.h:15,
                 from /vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/src/issuing-authority.cc:13:
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:46:31: error: ‘uint64_t’ has not been declared
   46 |     inline void setExpiryDate(uint64_t expiryDate)
      |                               ^~~~~~~~
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:51:30: error: ‘uint64_t’ has not been declared
   51 |     inline void setIssueDate(uint64_t issueDate)
      |                              ^~~~~~~~
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:81:12: error: ‘uint64_t’ does not name a type
   81 |     inline uint64_t expiryDate() const
      |            ^~~~~~~~
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:1:1: note: ‘uint64_t’ is defined in header ‘<cstdint>’; did you forget to ‘#include <cstdint>’?
  +++ |+#include <cstdint>
    1 | //
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:86:12: error: ‘uint64_t’ does not name a type
   86 |     inline uint64_t issueDate() const
      |            ^~~~~~~~
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:86:12: note: ‘uint64_t’ is defined in header ‘<cstdint>’; did you forget to ‘#include <cstdint>’?
/vcpkg/buildtrees/licensepp/src/612a3d3e3c-471c6e0659.clean/license++/license.h:123:5: error: ‘uint64_t’ does not name a type
  123 |     uint64_t m_issueDate;
      |     ^~~~~~~~