util/nvmutil: portable errno

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -431,6 +431,10 @@ static const struct commands command[] = {
  */
 static size_t cmd_index = CMD_NULL;
 
+#ifndef errno
+extern int errno;
+#endif
+
 typedef char assert_argc3[(ARGC_3==3)?1:-1];
 typedef char assert_argc4[(ARGC_4==4)?1:-1];