fix installpath on stretch+bionic

src/Makefile.xtables

@@ -6,11 +6,16 @@ clean:
 	rm -f $(LIBS)
 
 install:
-	if [ -d /usr/lib/x86_64-linux-gnu/xtables ]; then \
-      install -g root -o root -m 644 $(LIBS) /usr/lib/x86_64-linux-gnu/xtables; \
-    else \
-      install -g root -o root -m 644 $(LIBS) /lib/xtables/ ; \
-    fi
+	if [ -d /lib/xtables ]; then \
+	  install -g root -o root -m 644 $(LIBS) /lib/xtables/ ; \
+	elif [ -d /usr/lib/x86_64-linux-gnu/xtables ]; then \
+	  install -g root -o root -m 644 $(LIBS) /usr/lib/x86_64-linux-gnu/xtables; \
+	elif [ -d /usr/lib/i386-linux-gnu/xtables ]; then \
+	  install -g root -o root -m 644 $(LIBS) /usr/lib/i386-linux-gnu/xtables; \
+	else \
+	  echo "Unable to find xtables modules path!"
+	  exit 1
+	fi
 
 lib%.so: lib%.o
 	gcc -shared -fPIC -o $@ $^;