Prevents loading twice if using 'y_rconfix'

[MatheusAgL]

Fixes an error for those who use the YSI library

[Y-Less]

That won't work correctly, the define is only set when you include YSI, which must be after fixes.inc has been included. You'd need to check for the definition of another symbol like a function within y_rconfix. I'm also not sure if this is now needed as I added some extra checks to the code to detect the filterscript correctly (which should work between scripts as well).