classes/gtk-icon-cache: fix libdir passed to the postrm intercept

classes/gtk-icon-cache: fix libdir passed to the postrm intercept

Back in 2015[1] I fixed the libdir passed to the postinst intercept, but
I forgot to also update the postrm intercept.  This should also be
libdir_native, not libdir.

[ YOCTO #13896 ]

[1] oe-core 0fe8400717 ("gtk-icon-cache: pass the native libdir to the intercept")

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>