src/common/defs.h: add missing `<cstdint>` include (`gcc-13` fix)

[trofi]

Without the change build fails on gcc-13 as:

mapper/../common/defs.h:53:9: error: 'uint32_t' does not name a type
   53 | typedef uint32_t ref_pos_t;
      |         ^~~~~~~~
mapper/../common/defs.h:35:1: note: 'uint32_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
   34 | #include <string>
  +++ |+#include <cstdint>