utl: fix clang-tidy in mem_stat.h (include cstddef)

Signed-off-by: Matt Liberty <[email protected]>

Author: maliberty

src/utl/include/utl/mem_stats.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <cstddef>
+
 namespace utl {
 
 size_t getPeakRSS();