img_tools.pxi: Rename variable k
img_tools.pxi: Rename variable k

Brainfart, this was an int in the other PR, and I initially thought it
would not conflict. But it does, so use 'c2' for the char here, not 'k'.

matham/kivy,kivy/kivy,inclement/kivy,kivy/kivy,inclement/kivy,akshayaurora/kivy,matham/kivy,matham/kivy,matham/kivy,rnixx/kivy,akshayaurora/kivy,kivy/kivy,akshayaurora/kivy,rnixx/kivy,rnixx/kivy,inclement/kivy