package: Add pkg_postinst_ontarget to PACKAGEVARS

Changes to pkg_postinst_ontarget were not triggering rebuilds, this fixes
that.

[YOCTO #13127]

(From OE-Core rev: f3e2f78f282bf3a68a4923009825eb4fb460ca50)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/package.bbclass

@@ -2122,7 +2122,7 @@ python package_depchains() {
 
 # Since bitbake can't determine which variables are accessed during package
 # iteration, we need to list them here:
-PACKAGEVARS = "FILES RDEPENDS RRECOMMENDS SUMMARY DESCRIPTION RSUGGESTS RPROVIDES RCONFLICTS PKG ALLOW_EMPTY pkg_postinst pkg_postrm INITSCRIPT_NAME INITSCRIPT_PARAMS DEBIAN_NOAUTONAME ALTERNATIVE PKGE PKGV PKGR USERADD_PARAM GROUPADD_PARAM CONFFILES SYSTEMD_SERVICE LICENSE SECTION pkg_preinst pkg_prerm RREPLACES GROUPMEMS_PARAM SYSTEMD_AUTO_ENABLE SKIP_FILEDEPS PRIVATE_LIBS"
+PACKAGEVARS = "FILES RDEPENDS RRECOMMENDS SUMMARY DESCRIPTION RSUGGESTS RPROVIDES RCONFLICTS PKG ALLOW_EMPTY pkg_postinst pkg_postrm pkg_postinst_ontarget INITSCRIPT_NAME INITSCRIPT_PARAMS DEBIAN_NOAUTONAME ALTERNATIVE PKGE PKGV PKGR USERADD_PARAM GROUPADD_PARAM CONFFILES SYSTEMD_SERVICE LICENSE SECTION pkg_preinst pkg_prerm RREPLACES GROUPMEMS_PARAM SYSTEMD_AUTO_ENABLE SKIP_FILEDEPS PRIVATE_LIBS"
 
 def gen_packagevar(d):
     ret = []