fix build failures with gcc 13

[emollier]

Initially seen in Debian Bug#1037655, Filtlong fails to build with gcc 13. The relevant part of the build log looks like:

In file included from src/read.h:25,
                 from src/read.cpp:22:
src/kmers.h:37:26: error: ‘uint32_t’ has not been declared
   37 |     bool is_kmer_present(uint32_t kmer);
      |                          ^~~~~~~~
src/kmers.h:39:5: error: ‘uint32_t’ does not name a type
   39 |     uint32_t starting_kmer_to_bits_forward(char * sequence);
      |     ^~~~~~~~
src/kmers.h:25:1: note: ‘uint32_t’ is defined in header ‘<cstdint>’; did you forget to ‘#include <cstdint>’?

Adding cstdint to kmers.h per compiler hint allows the build to go through.