This seems to have been missed when the startup files were split out
during a refactor to support starting OpenRGB as a Windows service in
cf2df815e3.
Without this change, we see `ld: error: undefined symbol: main` when
compiling on FreeBSD.
I chose to name the file main_FreeBSD_Linux_MacOS instead of
main_Linux_MacOS_FreeBSD to keep the list alphabetical.