Add `mergesort.c` to files to compile in GitHub Actions#14

NicMcPhee merged 4 commits intomainUMM-CSci-Systems/C-programming-arrays:mainfrom

add-mergesort-to-arraymerge-compileUMM-CSci-Systems/C-programming-arrays:add-mergesort-to-arraymerge-compileCopy head branch name to clipboard