labels: Add missing backend type for extern symbols

Typo in 98578071b9

https://bugzilla.nasm.us/show_bug.cgi?id=3392494

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

asm/labels.c

@@ -175,6 +175,7 @@ static void out_symdef(union label *lptr)
     /* Clean up this hack... */
     switch(lptr->defn.type) {
     case LBL_GLOBAL:
+    case LBL_EXTERN:
         backend_type = 1;
         backend_offset = lptr->defn.offset;
         break;