package/zfs: replace PYTHON_VERSION_MAJOR

Replace PYTHON_VERSION_MAJOR by PYTHON3_VERSION_MAJOR now that python2
has been dropped. It should be noted that PYTHON_SITE_PKG was wrongly
set since the addition of the package in commit
4470bc9914d730383db86f58bc86fb09739622dc

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/zfs/zfs.mk

@@ -44,7 +44,7 @@ ZFS_CONF_ENV += \
 	PYTHON_CPPFLAGS="`$(STAGING_DIR)/usr/bin/python3-config --includes`" \
 	PYTHON_LIBS="`$(STAGING_DIR)/usr/bin/python3-config --ldflags`" \
 	PYTHON_EXTRA_LIBS="`$(STAGING_DIR)/usr/bin/python3-config --libs --embed`" \
-	PYTHON_SITE_PKG="/usr/lib/python$(PYTHON_VERSION_MAJOR)/site-packages"
+	PYTHON_SITE_PKG="/usr/lib/python$(PYTHON3_VERSION_MAJOR)/site-packages"
 ZFS_CONF_OPTS += --enable-pyzfs
 else
 ZFS_CONF_OPTS += --disable-pyzfs --without-python