remove unused includes

This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.

src/path/path.c

@@ -22,9 +22,7 @@
 #include <stdio.h>
 #include <getopt.h>
 #include <errno.h>
-#include <unistd.h>
 #include <stdlib.h>
-#include <string.h>
 
 #include "sd-path.h"
 #include "build.h"