skeleton/service: Ensure debug path handling works as intended

DEBUG_PREFIX_MAP uses ${S} but that wasn't set correctly for this
recipe meaning cwd during the build (WORKDIR) was encoded into the
binary leading to buildpath warnings in debug symbols. Set S correctly
to avoid this issue.

(From OE-Core rev: 299abc1d69db13924f02d922139051126f19b306)

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

meta-skeleton/recipes-skeleton/service/service_0.1.bb

@@ -9,6 +9,8 @@ SRC_URI = "file://skeleton \
 	   file://COPYRIGHT \
 	   "
 
+S = "${WORKDIR}"
+
 do_compile () {
 	${CC} ${CFLAGS} ${LDFLAGS} ${WORKDIR}/skeleton_test.c -o ${WORKDIR}/skeleton-test
 }