ev_arc4random_buf: fix 'return arc4random_buf();' to use Clang

Return type of ev_arc4random_buf() is void, so it has to call first
arc4random_buf() and then return.
Compiling with Clang, this would have caused an error, while with GCC only a
warning.
Merge pull request #2 from alessandro40/patches-2.0

`return arc4random_buf()` => `arc4random_buf(); return` to make clang happy