src: include cstddef in headers that use size_t

src/native/skate3_native_diag.h

@@ -9,6 +9,7 @@
 // TU.
 
 #include <atomic>
+#include <cstddef>
 #include <cstdint>
 #include <memory>
 #include <mutex>

src/skate3_native_scene.h

@@ -7,6 +7,7 @@
 // guest output texture via the SDK native-guest-output hook, replacing the
 // emulated frame when active.
 
+#include <cstddef>
 #include <cstdint>
 #include <vector>
 

src/skate3_native_scene_state.h

@@ -9,6 +9,7 @@
 #include <array>
 #include <atomic>
 #include <chrono>
+#include <cstddef>
 #include <condition_variable>
 #include <cstdint>
 #include <cstdio>

src/skate3_ultrawide_guest.h

@@ -3,6 +3,7 @@
 #include <algorithm>
 #include <array>
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 
 #include <rex/cvar.h>