src: introduce unreleased export section

It is not obvious to start a new section after a release. Add an
explicit new UNRELEASED section so contributers know where to put a new
symbol.

Signed-off-by: Daniel Wagner <[email protected]>

Author: igaw

src/libnvme-mi.map

@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+LIBNVME_UNRELEASED {
+};
+
 LIBNVME_MI_1_12 {
 	global:
 		nvme_mi_mi_xfer;

src/libnvme.map

@@ -1,4 +1,7 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
+LIBNVME_UNRELEASED {
+};
+
 LIBNVME_1.13 {
 	global:
 		nvme_subsystem_get_fw_rev;