fix signedness warning

src/core/disk_ops/disk_stdio.c

@@ -44,7 +44,7 @@
 #define TEMP_HIERARCHY_DEPTH 2
 
 /* ------------------- use STDIO to write on disk -------------------  */
-static int starpu_stdio_opened_files;
+static unsigned starpu_stdio_opened_files;
 
 struct starpu_stdio_obj
 {

src/core/disk_ops/unistd/disk_unistd_global.c

@@ -53,7 +53,7 @@
 
 /* TODO: on Linux, use io_submit */
 
-static int starpu_unistd_opened_files;
+static unsigned starpu_unistd_opened_files;
 
 #ifdef HAVE_AIO_H
 struct starpu_unistd_aiocb {