Add missing direct include to alloc.h

Silences an LSP error.

Author: gtaylor

src/mux/support/alloc.h

@@ -2,6 +2,8 @@
 
 #pragma once
 
+#include <stdlib.h>
+
 #include "mux/support/stringutil.h"
 
 constexpr int LBUF_SIZE = 16384;