iscsi-initiator-utils: Avoid hardcoding sed to /usr/bin/sed

This ensures that it can use the sed provided by build environment, as
we poison host sysroots, we wont be able to get it from /usr/bin anyway

Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-networking/recipes-daemons/iscsi-initiator-utils/iscsi-initiator-utils_2.1.7.bb

@@ -42,6 +42,7 @@ EXTRA_OEMAKE = ' \
     MANDIR="${mandir}" \
     OPTFLAGS="-DNO_SYSTEMD ${CFLAGS}" \
     PKG_CONFIG="${STAGING_BINDIR_NATIVE}/pkg-config" \
+    SED=sed \
 '