harfbuzz: remove bindir only if it exists

In some scenarios (e.g. when "glib" removed from PACKAGECONFIG),
"${D}${bindir}" might not exist which caused `rmdir` to fail.

Signed-off-by: Petr Kubizňák <kubiznak@2n.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>