[rom_ext] Mark static critical version as used

The previous PR missed updating this file.

Marking it as used prevents optimizations such as LTO from stripping
the value.

Change-Id: I60bac0f10d35e59f17bde24b6d60c5f8130b5cb4
Signed-off-by: Yi-Hsuan Deng <yhdeng@google.com>

Author: sasdf

sw/device/silicon_creator/lib/base/static_critical_version.c

@@ -14,4 +14,4 @@
  * populated by the ROM before the jump to ROM_EXT.
  */
 OT_SET_BSS_SECTION(".static_critical.version",
-                   uint32_t static_critical_version;)
+                   OT_USED uint32_t static_critical_version;)