[cairo] Fix OS/2 build

[Behdad Esfahbod]

Peter Weilbacher wrote:

> I often get the impression that you don't want OS/2 people to use cairo.
> Before we broken when compiling cairo, now we break when compiling most
> cairo apps on OS/2. Not sure this is an improvement.

Nothing against OS/2 specifically.  The cairo code base moves and the backends
need to move along.  I do my best to update code for all backends, but not
being able to compile many of them, things break and we rely on people testing
them to report back.

In this particular case, it's our policy for all supported backends.  I'll
remove the include from cairo-os2.h if you want me to, but that just adds one
more obstacle in front of the OS/2 to become an officially supported cairo
backend one day.

behdad

>    Peter.