all: free() is defined in stdlib.h

Without including <stdlib.h> newer gcc releases throw
warnings about undefined free().

src/disk.c

@@ -19,6 +19,7 @@
 #include <fcntl.h>
 #include <libgen.h>
 #include <unistd.h>
+#include <stdlib.h>
 
 #include <sys/types.h>
 #include <sys/stat.h>

src/operations.c

@@ -17,6 +17,8 @@
 
 */
 
+#include <stdlib.h>
+
 #include "global.h"
 #include "device.h"
 #include "usb-device.h"