HDFS-10991. Export hdfsTruncateFile symbol in libhdfs. Contributed by Surendra Singh Lilhore.

(cherry picked from commit dacd3ec66b111be24131957c986f0c748cf9ea26)

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfs/include/hdfs/hdfs.h

@@ -408,6 +408,7 @@ extern  "C" {
      *         complete before proceeding with further file updates.
      *         -1 on error.
      */
+    LIBHDFS_EXTERNAL
     int hdfsTruncateFile(hdfsFS fs, const char* path, tOffset newlength);
 
     /**