package: python3-jinja2: Fix for including non-existent file

The included mk/python.mk was removed by commit 179b687e2f99d ("remove
python2 dependencies and packages"). Including python3.mk might be the
right alternative?

Signed-off-by: Phil Sutter <phil@nwl.cc>

package/python3-jinja2/Makefile

@@ -22,7 +22,7 @@ include $(ADK_TOPDIR)/mk/package.mk
 
 $(eval $(call HOST_template,PYTHON3_JINJA2,python3-jinja2,${PKG_VERSION}-${PKG_RELEASE}))
 
-include $(ADK_TOPDIR)/mk/python.mk
+include $(ADK_TOPDIR)/mk/python3.mk
 
 HOST_STYLE:=		manual