Change Makefile indentation character to tab

Per [review comment by
@pavoljuhas](#885 (comment)),
Makefiles use tab for indentation and so the configuration needs to
change `indent_style` accordingly.

Author: mhucka

.editorconfig

@@ -39,6 +39,7 @@ indent_size = 4
 indent_size = 4
 
 [{Makefile,makefile,*.mk}]
+indent_style = tab
 indent_size = 4
 
 [{*.cc,*.h}]