mut_get_ins_long_n: Not inline

Fix this error:
gcc -g -Wall -O2  -o dwgsim src/dwgsim_opt.o src/mut.o src/contigs.o src/regions_bed.o src/mut_txt.o src/mut_bed.o src/mut_vcf.o src/mut_input.o src/dwgsim.o -lm -lz
Undefined symbols for architecture x86_64:
  "_mut_get_ins_long_n", referenced from:
      _dwgsim_core in dwgsim.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)