Add a missing semicolon in src/io.c.

[allevato]

I'm really confused about why this doesn't fail to compile on swift-ci; I would expect that #else branch to be tested when building for Linux. When I recently updated our internal copy of the Linux toolchain past https://github.com/apple/swift-corelibs-libdispatch/pull/805, it failed:

src/io.c:2377:59: error: expected ';' after expression
 2377 |                         op->buf = aligned_alloc((size_t)PAGE_SIZE, op->buf_siz)
      |                                                                                ^
      |                                                                                ;

[allevato]

@swift-ci please test

[allevato]

@compnerd I don't have write access on this repo; can you please take a look? 🙂

[finagolfin]

This will need to be fixed in the 6.0 branch also.

[allevato]

This will need to be fixed in the 6.0 branch also.

https://github.com/apple/swift-corelibs-libdispatch/pull/824