In DosGlob.xs, add a block around MY_CXT_INIT, as it contains a declaration.

In DosGlob.xs, add a block around MY_CXT_INIT, as it contains a declaration.

The macro MY_CXT_INIT contains a variable declaration, and xsubpp doesn't
wrap XS BOOT sections within a block, hence code at the start of the BOOT
section may already be after code. Hence wrap MY_CXT_INIT within a block, to
ensure that its initialisation is before any code.