packages/utils: fbtest fix Makefile

The clean target tries to remove what looks like a bogus 'rbcfg',
probably carried over copy-pasta. Remove the name of the generated
executable ('fbtest') instead.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
Fixes: 8099f4e0d3 ("fbtest utility ")

package/utils/fbtest/src/Makefile

@@ -11,4 +11,4 @@ fbtest: $(OBJS)
 	$(CC) -o $@ $(OBJS)
 
 clean:
-	rm -f rbcfg *.o
+	rm -f fbtest *.o