Avoid a compiler warning about a potentially unused variable.

Avoid a compiler warning about a potentially unused variable.

There is no assignment to the "ret" variable if X_DRI2SwapBuffers is
not defined. In this case, the earlier explicit "return 0" is likely
to be used, but the compiler can't be sure of that, (nor can I for
that matter).

We cover this case by explicitly initializing "ret" to 0.