analyze: include unistd.h

The source file uses symbols e.g. execl(), execvp(), _exit(), and so on,
without including unistd.h.

Continuation of 4f18ff2e29.

Follow-up for 9a08000d18.

src/analyze/analyze-unit-shell.c

@@ -2,6 +2,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "sd-bus.h"