Add Makefile-ponyc to the Appveyor ignore list (#3041)
Add Makefile-ponyc to the Appveyor ignore list (#3041)

Changes to that Makefile have no impact on the Windows build and
shouldn't trigger a build.

[skip  ci]
ponylang/ponyc,ponylang/ponyc,kulibali/ponyc,sgebbie/ponyc,dipinhora/ponyc,cquinn/ponyc,dipinhora/ponyc,dipinhora/ponyc,CausalityLtd/ponyc,ponylang/ponyc,cquinn/ponyc,CausalityLtd/ponyc,kulibali/ponyc,sgebbie/ponyc,jemc/ponyc,sgebbie/ponyc,sgebbie/ponyc,jemc/ponyc,jemc/ponyc,kulibali/ponyc,cquinn/ponyc,cquinn/ponyc,kulibali/ponyc,CausalityLtd/ponyc,sgebbie/ponyc