extract/ExtNghbors.c: fix older compiler support for label

ExtNghbors.c:615:1: error: label at end of compound statement

since: 846c8e0f6 (2026-01-15 8.3.591)

This fixes AppImage 7/8/9 which use older compiler versions.

extract/ExtNghbors.c

@@ -614,6 +614,7 @@ termtop:
 
 termdone:
 	/* (continue) */
+	(void)0; /* older compilers need a statement after the label to prevent a compile error */
     }
     
     /* Clean up---Put the ClientData entries in the tiles back to