mm/mempool: fix visual studio Compiler Warning C4098

D:\archer\code\nuttx\mm\mempool\mempool_multiple.c(180,72): warning C4098: "mempool_multiple_free_callback":"void" void function returning a value

Compiler Warning C4098:
A function declared with return type void has a return statement that returns a value. The compiler assumes the function returns a value of type int.

Reference:
https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4098?view=msvc-170

Signed-off-by: chao an <anchao@xiaomi.com>

Author: anchao

mm/mempool/mempool_multiple.c

@@ -177,7 +177,7 @@ static void mempool_multiple_free_callback(FAR struct mempool_s *pool,
 {
   FAR struct mempool_multiple_s *mpool = pool->priv;
 
-  return mpool->free(mpool->arg, (FAR char *)addr - mpool->minpoolsize);
+  mpool->free(mpool->arg, (FAR char *)addr - mpool->minpoolsize);
 }
 
 /****************************************************************************