unix: fix compiler warning in async.c

Include missing <string.h> header. Fixes the following compiler warning:

  src/unix/async.c:182:7: warning: implicit declaration of
  function ‘memmove’ [-Wimplicit-function-declaration]

src/unix/async.c

@@ -24,6 +24,7 @@
 #include <errno.h>
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 
 static int uv__async_init(uv_loop_t* loop);