bootm: vxworks: Make do_bootm_vxworks() non-static

For future extension to other architectures, make do_bootm_vxworks()
a non-static function.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/bootm.h b/include/bootm.h
index 0501414..dbd6f49 100644
--- a/include/bootm.h
+++ b/include/bootm.h
@@ -35,6 +35,8 @@
 			bootm_headers_t *images);
 
 extern boot_os_fn do_bootm_linux;
+extern boot_os_fn do_bootm_vxworks;
+
 int do_bootelf(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[]);
 void lynxkdi_boot(image_header_t *hdr);