package/rapidxml: fix install path

package/rapidxml: fix install path

kodi-pvr-iptvsimple as of
https://github.com/kodi-pvr/pvr.iptvsimple/commit/d27b3ed1d379d2f865e95f08bdf2dbf086f8c0ad

depends on rapidxml but expects the headers in usr/include/rapidxml
instead of usr/include.

This changes the install path to be consistent with Debian and Gentoo:

https://packages.debian.org/stretch/all/librapidxml-dev/filelist
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-libs/rapidxml/rapidxml-1.13-r1.ebuild#n22

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>