Update rsrc.go

I did a little change to allow add more icon files using a dirty way . and print the ID of files added .
icon files using a comma( , ) to separate Can be use like this 

rsrc.exe -manifest rsrc.manifest -ico x.ico,y.ico 

The output is 

Manifest ID :  1
Icon  x.ico  ID :  3
Icon  y.ico  ID :  5

You can use it on [walk](https://github.com/lxn/walk) by 
x.ico NewIconFromResource("3") 
y.ico NewIconFromResource("5")