Add .editorconfig file

Many project in the freedesktop graphics space use editorconfig files as
a way to standardize style in an editor agnostic way. This one
configures all of the files current in libglvnd (I looked at the .c and
.h files and they seem to all use 4 spaces for indent). Because it
seemed everything used 4 spaces I just set that to the default and
overrode as necessary.