Use size_t for the mjMARKSTACK temporary variable.

This is necessary since daa2ac7f8b changed the type of mjData pstack from int to size_t.

PiperOrigin-RevId: 561092753
Change-Id: I298507c2c0656c797637b35ee684b73eefaae8f5

include/mujoco/mjmacro.h

@@ -15,6 +15,8 @@
 #ifndef MUJOCO_MJMACRO_H_
 #define MUJOCO_MJMACRO_H_
 
+#include <stddef.h>
+
 // include asan interface header, or provide stubs for poison/unpoison macros when not using asan
 #ifdef ADDRESS_SANITIZER
   #include <sanitizer/asan_interface.h>
@@ -31,7 +33,7 @@
 #define mjMIN(a, b) (((a) < (b)) ? (a) : (b))
 
 // mjData stack frame management
-#define mjMARKSTACK   int _mark = d->pstack;
+#define mjMARKSTACK   size_t _mark = d->pstack;
 #define mjFREESTACK   d->pstack = _mark;
 
 // return current value of mjOption enable/disable flags