On the ^/subversion/branches/1.8.x-r1537193 branch: Fix build.

* subversion/tests/svn_test_main.c
  (): add missing #include

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1537193@1540428 13f79535-47bb-0310-9956-ffa450edef68