add missing <optional> header to src/projects/mts/kmer_multiplicity_counter.cpp

[douglasgscofield]

src/projects/mts/kmer_multiplicity_counter.cpp is missing #include <optional>.

Compiling with gcc/12.3.0 or /13.3.0, its lack results in errors like

/sw/bioinfo/spades/4.0.0/src/SPAdes-4.0.0/src/projects/mts/kmer_multiplicity_counter.cpp:104:18: error: 'optional' is not a member of 'std'
  104 |             std::optional<RtSeq> min_kmer;
      |                  ^~~~~~~~
/sw/bioinfo/spades/4.0.0/src/SPAdes-4.0.0/src/projects/mts/kmer_multiplicity_counter.cpp:27:1: note: 'std::optional' is defined in header '<optional>'; did you forget to '#include <optional>'?

[asl]

@douglasgscofield Thanks! How you're building SPAdes? mts should not be enabled by default. Are you building all projects?

[douglasgscofield]

Yes, that's exactly it