shadow.inc: run postinst only for target

* fails for nativesdk-shadow with:
  pwconv: /etc/passwd.29063: No such file or directory
  pwconv: cannot lock /etc/passwd; try again later.

(From OE-Core rev: c292945611d9f825051ac4938bb22a7d42fff994)

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

meta/recipes-extended/shadow/shadow.inc

@@ -190,7 +190,7 @@ ALTERNATIVE_LINK_NAME[su.1] = "${mandir}/man1/su.1"
 ALTERNATIVE_LINK_NAME[nologin.8] = "${mandir}/man8/nologin.8"
 
 PACKAGE_WRITE_DEPS += "shadow-native"
-pkg_postinst_${PN} () {
+pkg_postinst_${PN}_class-target () {
 	if [ "x$D" != "x" ]; then
 	  rootarg="--root $D"
 	else