dwm: Add rpath promise

Web browsers with plugins such as uBlock Origin that block remote fonts
will trigger dwm to load fonts locally.

The "rpath" promise is currently missing since dwm doesn't read any
files for itself at runtime.

Reported and pinned down by a user.

OK?

Index: Makefile
===================================================================
RCS file: /cvs/ports/x11/dwm/Makefile,v
retrieving revision 1.29
diff -u -p -r1.29 Makefile
--- Makefile 22 May 2018 13:30:56 -0000 1.29
+++ Makefile 2 Jun 2018 11:30:23 -0000
@@ -4,7 +4,7 @@ COMMENT= dynamic window manager

V= 6.1
DISTNAME= dwm-${V}
-REVISION= 1
+REVISION= 2

CATEGORIES= x11

Index: patches/patch-dwm_c
===================================================================
RCS file: /cvs/ports/x11/dwm/patches/patch-dwm_c,v
retrieving revision 1.1
diff -u -p -r1.1 patch-dwm_c
--- patches/patch-dwm_c 22 May 2018 13:30:57 -0000 1.1
+++ patches/patch-dwm_c 2 Jun 2018 11:31:05 -0000
@@ -7,7 +7,7 @@ Index: dwm.c
die("dwm: cannot open display\n");
checkotherwm();
setup();
-+ if (pledge("stdio proc exec", NULL) == -1)
++ if (pledge("stdio rpath proc exec", NULL) == -1)
+ die("pledge\n");
scan();
run();