atomics: Fix the win32 atomic_exchange function

This fixes building with MSVC after
a2a38b160620d91bc3f895dadc4501c589998b9c.

Remove the stray semicolon, and add casts for the input argument
(which is an intptr_t*) to the right type (PVOID volatile *).

Signed-off-by: Martin Storsjö <martin@martin.st>
