gh-90815: Fix mimalloc build on WASI

Include <unistd.h> to get sbrk() function.

Author: vstinner

Objects/mimalloc/prim/wasi/prim.c

@@ -11,6 +11,7 @@ terms of the MIT license. A copy of the license can be found in the file
 #include "mimalloc/internal.h"
 #include "mimalloc/atomic.h"
 #include "mimalloc/prim.h"
+#include <unistd.h>               // sbrk()
 
 //---------------------------------------------
 // Initialize