minimalist-pcproxy: fix post install script

Fixes:
Configuring minimalist-pcproxy.
sh: 1: unknown operand

Reported-by: Daniel Golle <daniel@makrotopia.org>
Suggested-by: Daniel Golle <daniel@makrotopia.org>

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

minimalist-pcproxy/files/minimalist-pcproxy.defaults

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-if [ `uci -q get upnpd.config._pcproxy_configured` = "1" ]
+if [ "`uci -q get upnpd.config._pcproxy_configured`" = "1" ]
 then
     exit
 fi