useradd-example: Fix S = WORKDIR reference

Update not to use S = WORKDIR after that was no longer allowed.

(From OE-Core rev: 50f78cb9de68cd4317f34321dfdb06d72ce5d3c6)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-skeleton/recipes-skeleton/useradd/useradd-example.bb

@@ -9,7 +9,8 @@ SRC_URI = "file://file1 \
            file://file3 \
            file://file4"
 
-S = "${WORKDIR}"
+S = "${WORKDIR}/sources"
+UNPACKDIR = "${S}"
 
 PACKAGES =+ "${PN}-user3"