add pledge(2) to "mutool poster"

Index: source/tools/pdfposter.c
--- source/tools/pdfposter.c.orig
+++ source/tools/pdfposter.c
@@ -27,6 +27,8 @@
 #include "mupdf/fitz.h"
 #include "mupdf/pdf.h"
 
+#include <unistd.h>
+#include <errno.h>
 #include <string.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -233,6 +235,12 @@ int pdfposter_main(int argc, char **argv)
 		(strstr(argv[fz_optind], ".pdf") || strstr(argv[fz_optind], ".PDF")))
 	{
 		outfile = argv[fz_optind++];
+	}
+
+	if (pledge("stdio rpath wpath cpath", NULL) == -1)
+	{
+		fprintf(stderr, "pledge: %s\n", strerror(errno));
+		exit(1);
 	}
 
 	ctx = fz_new_context(NULL, NULL, FZ_STORE_UNLIMITED);