[SPARK-11755][R] SparkR should export "predict"

The bug described at [SPARK-11755](https://issues.apache.org/jira/browse/SPARK-11755), after exporting ```predict``` we can both get the help information from the SparkR and base R package like the following:
```Java
> help(predict)
Help on topic ‘predict’ was found in the following packages:

  Package               Library
  SparkR                /Users/yanboliang/data/trunk2/spark/R/lib
  stats                 /Library/Frameworks/R.framework/Versions/3.2/Resources/library

Choose one

1: Make predictions from a model {SparkR}
2: Model Predictions {stats}
```

Author: Yanbo Liang <[email protected]>

Closes #9732 from yanboliang/spark-11755.

(cherry picked from commit 8fb775b)
Signed-off-by: Shivaram Venkataraman <[email protected]>

Author: yanboliang

R/pkg/R/generics.R

@@ -1054,6 +1054,10 @@ setGeneric("year", function(x) { standardGeneric("year") })
 #' @export
 setGeneric("glm")
 
+#' @rdname predict
+#' @export
+setGeneric("predict", function(object, ...) { standardGeneric("predict") })
+
 #' @rdname rbind
 #' @export
 setGeneric("rbind", signature = "...")