package/pkg-meson.mk: fix cross-compilation.conf

package/pkg-meson.mk: fix cross-compilation.conf

$$(STAGING_DIR) -> $(STAGING_DIR) in PKG_MESON_INSTALL_CROSS_CONF.

$$ resulted in `$(STAGING_DIR)` in the file instead of the expanded
value.

Note that this change only affects the etc config at:
host/etc/meson/cross-compilation.conf

Per-package cross-compilation.conf files are already correct.

Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Gleb Mazovetskiy <glex.spb@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>