Add support for AIX

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/os/os.h b/os/os.h
index 218766a..e2bb0ba 100644
--- a/os/os.h
+++ b/os/os.h
@@ -14,6 +14,8 @@
 #include "os-solaris.h"
 #elif defined(__APPLE__)
 #include "os-mac.h"
+#elif defined(_AIX)
+#include "os-aix.h"
 #else
 #error "unsupported os"
 #endif