log: Fix build on musl libc

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>

qdevices/log-common.c

@@ -32,6 +32,7 @@
  * THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <stddef.h>
 #include "log.h"
 #include "log-common.h"
 #include "utils.h"