Use rm -df instead of rmdir -p since rmdir requires the directory exist
so it causes "make -C tools clean" failed if someone only builds other
tools but not counter.

Fixes: 228354ed ("tools/counter: Makefile: Remove lingering 'include' directories on make clean")
Signed-off-by: Anh Tuan Phan <tuananhlfc@gmail.com>
Link: https://lore.kernel.org/r/d4080db5-1825-2848-079a-8bb674d8ee44@gmail.com/Signed-off-by: William Breathitt Gray <william.gray@linaro.org>