add missing <cstdint> include

[trofi]

Upcoming gcc-13 made <string> leaner and does not include <cstdint> implicitly anymore. As a result build fails without the change as:

[  2%] Building CXX object CMakeFiles/wayland-scanner++.dir/scanner/scanner.cpp.o
scanner/scanner.cpp:378:3: error: 'uint32_t' does not name a type
  378 |   uint32_t width = 0;
      |   ^~~~~~~~

[heitbaum]

Tested as working with gcc-13.1. 👍

@NilsBrause - would it be possible to merge and potentially issue a 1.0.1 release