Fix very minor Makefile issue in spidermonkey clean target (#109)

[mzpqnxow]

This fixes #109 and while I didn't test it across different platforms, I did confirm there is no breakage with the build or clean targets on a Linux system with a glibc+gcc toolchain

If you don't think this is really necessary please feel free to close it

Thanks!

[mzpqnxow]

@manugarg any interest in merging this? Or maybe you committed a fix yourself already? I'm just testing out the GitHub mobile app, apologies if you already fixed this, I vaguely remember you commenting on this but I don't see it here

[manugarg]

Thanks for this. Sorry for the super long delay.