clang-tidy: Apply modernize-deprecated-headers

This can be reproduced according to the developer notes with something
like

( cd ./src/ && ../contrib/devtools/run-clang-tidy.py -p ../bld-cmake -fix -j $(nproc) )

Also, the header related changes were done manually.

src/bench/examples.cpp

@@ -5,7 +5,7 @@
 #include <bench/bench.h>
 
 // Extremely fast-running benchmark:
-#include <math.h>
+#include <cmath>
 
 volatile double sum = 0.0; // volatile, global so not optimized away
 

src/bench/parse_hex.cpp

@@ -4,7 +4,7 @@
 
 #include <bench/bench.h>
 #include <random.h>
-#include <stddef.h>
+#include <cstddef>
 #include <util/strencodings.h>
 #include <cassert>
 #include <optional>