Fix the travis issue

For example [1]:

    FAILURE: Build failed with an exception.
    * Where:
    Script '/thrift/src/thrift-0.17.0/lib/java/gradle/generateTestThrift.gradle' line: 40
    * What went wrong:
    A problem occurred evaluating script.
    > assert thriftFile.exists()
             |          |
             |          false
             /thrift/src/thrift-0.17.0/test/partial/thrift_test_schema.thrift

[1]: https://app.travis-ci.com/github/apache/thrift/jobs/557030315
diff --git a/test/Makefile.am b/test/Makefile.am
index 15e2f1b..5b9c1ce 100755
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -166,6 +166,7 @@
 	UnsafeTypes.thrift \
 	Service.thrift \
 	SpecificNameTest.thrift \
+	partial/thrift_test_schema.thrift \
 	known_failures_Linux.json \
 	test.py \
 	tests.json \