fix build error in CI

Author: bhaller

core/population.cpp

@@ -7787,7 +7787,7 @@ void Population::RemoveAllFixedMutations(void)
 			// When recording tree-sequence mutations, mutations that have been lost or removed need to be
 			// "retained by the tree sequence" since they are still present in derived states, so that their
 			// metadata is preserved.  The tree sequence thus takes over their retain here.
-			std::vector<MutationIndex> &muts_tracked_by_treeseq = species_.muts_tracked_by_treeseq_;
+			std::vector<MutationIndex> &muts_tracked_by_treeseq = species_.MutationsTrackedByTreeSeq();
 
 			for (int i = 0; i < removed_mutation_accumulator.size(); i++)
 			{

core/species.h

@@ -782,6 +782,8 @@ class Species : public EidosDictionaryUnretained
 	slim_tick_t _InitializePopulationFromTskitBinaryFile(const char *p_file, EidosInterpreter *p_interpreter, SUBPOP_REMAP_HASH &p_subpop_remap, Chromosome &p_chromosome);	// initialize the population from an tskit binary file
 	slim_tick_t _InitializePopulationFromTskitDirectory(std::string p_directory, EidosInterpreter *p_interpreter, SUBPOP_REMAP_HASH &p_subpop_remap);	// initialize the population from a multi-chromosome directory
 
+	std::vector<MutationIndex> &MutationsTrackedByTreeSeq(void) { return muts_tracked_by_treeseq_; }
+	
 	size_t MemoryUsageForTreeSeqInfo(TreeSeqInfo &p_tsinfo, bool p_count_shared_tables);
 	void TSXC_Enable(void);
 	void TSF_Enable(void);