util: xrealloc should take a size_t

For correctness only because this is C and types are mostly
just there for lolz, not because they're actualy useful.

Assisted-by: Claude:claude-opus-4-6
Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/388>

src/util-mem.h

@@ -117,7 +117,7 @@ xalloc(size_t size)
 }
 
 static inline void *
-xrealloc(void *ptr, int size)
+xrealloc(void *ptr, size_t size)
 {
 	void *tmp = realloc(ptr, size);
 	assert(tmp);