package/pkg-meson: really generate the global cross-compilation.conf

Commit ccc9e059 ("package/meson:
install cross-compilation.conf during toolchain install") moved the
logic to generate the global cross-compilation.conf from
package/meson/meson.mk to package/pkg-meson.mk. While doing so, it
renamed the macro from HOST_MESON_INSTALL_CROSS_CONF to
PKG_MESON_INSTALL_CROSS_CONF, but the registration of this hook in
TOOLCHAIN_POST_INSTALL_STAGING_HOOKS was not changed accordingly: it
is still registering HOST_MESON_INSTALL_CROSS_CONF.

Due to this, the global cross-compilation.conf file was no longer
generated.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>