Use C++ includes

src/build.cpp

@@ -5,9 +5,9 @@
 
 #include "fs.hpp"
 #include <sstream>
-#include <stddef.h>
+#include <cstddef>
 #include <stdexcept>
-#include <stdlib.h>
+#include <cstdlib>
 #include <system_error>
 
 namespace cmkr {

src/error.cpp

@@ -1,6 +1,6 @@
 #include "error.hpp"
 
-#include <assert.h>
+#include <cassert>
 
 namespace cmkr {
 namespace error {