Fix liburing.h memset() missing declaration

liburing.h uses memset() but doesn't include string.h.
If nothing else includes it, this causes compile errors.
Add the include.

Signed-off-by: Ian Gulliver <git@flamingcow.io>

src/liburing.h

@@ -7,6 +7,7 @@ extern "C" {
 
 #include <sys/uio.h>
 #include <signal.h>
+#include <string.h>
 #include <inttypes.h>
 #include "compat.h"
 #include "io_uring.h"