proot: Only bind /property_contexts if file exists

Fixes the following warning when running termux-chroot on Android 8:

proot warning: can't sanitize binding "/property_contexts":
No such file or directory

packages/proot/termux-chroot

@@ -31,8 +31,11 @@ ARGS="$ARGS -b /vendor:/vendor"
 # and $HOME so that Termux programs with hard-coded paths continue to work:
 ARGS="$ARGS -b /data:/data"
 
-# Used by getprop (see https://github.com/termux/termux-packages/issues/1076):
-ARGS="$ARGS -b /property_contexts:/property_contexts"
+if [ -f /property_contexts ]; then
+	# Used by getprop (see https://github.com/termux/termux-packages/issues/1076)
+	# but does not exist on Android 8.
+	ARGS="$ARGS -b /property_contexts:/property_contexts"
+fi
 
 # Expose external and internal storage:
 if [ -d /storage ]; then