Makefile: Change deprecated Inkscape flag
    
    This was annoying to figure out because the Inkscape manpage still shows
    `--export-png` as a valid option, even though it was removed in the last
    release. It has to be `--export-file` now:
    
        https://wiki.inkscape.org/wiki/index.php/Using_the_Command_Line#New_options
    
    Undocumented features…
    
    Since I still use this Makefile for my own blog featured images, I
    figured I should go ahead and upstream my patch. Thanks @pfrields and
    @ryanlerch for this handy little script:
    
        https://github.com/jwflory/blog-images/commit/937c3d3627e82536cdc31dc73af6aa50d2108d51
    
    Signed-off-by: Justin W. Flory <git@jwf.io>