[HAVE_ST_BLOCKS || _POSIX_VERSION]: Add extern dcl

of unused variable so that after preprocessing, the file will never
be empty.  Otherwise, some compilers in ANSI mode (among them, the
Dec Alpha OSF 2.0 C compiler) reject empty files.  From Kaveh Ghazi.

lib/fileblocks.c

@@ -63,4 +63,8 @@ st_blocks (size)
 
   return datablks + indrblks;
 }
+#else
+/* This declaration is solely to ensure that after preprocessing
+   this file is never empty.  */
+extern int textutils_fileblocks_unused;
 #endif