Add cstring header explictly as it is removed from HIP (#1859)

[ROCm/rccl commit: 6d41e5ba99]

projects/rccl/src/include/utils.h

@@ -12,6 +12,7 @@
 #include "bitops.h"
 #include "checks.h"
 #include <stdint.h>
+#include <string.h>
 #include <time.h>
 #include <sched.h>
 #include <algorithm>