dissect: include unistd.h

This is required by e.g. geteuid(), for the case when unistd.h is
indirectly included by other headers.

Continuation of 4f18ff2e29.

src/dissect/dissect.c

@@ -6,6 +6,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/file.h>
+#include <unistd.h>
 
 #include "sd-device.h"