libical: improve buildpath situation

libical: improve buildpath situation

Take an upstream patch to remove buildpaths from the generator's CMake
files, so we don't need to sed them out.

Also, the generator tool is only needed when cross-compiling libical,
so we don't actually need to install it in the target package. This has
the nice side-effect of removing libxml2 from the target RDEPENDS as it
is only used in the tool.

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>