<fts.h>: fix nullability annotation.

The array of paths is NULL-terminated. Add a basic smoke test for
<fts.h>, but our clang isn't clever enough to notice this particular
header file bug anyway.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>
Test: treehugger
Change-Id: I6afa93e8a2a7edf64dd54240a98d7bd6c13283a8
diff --git a/tests/Android.bp b/tests/Android.bp
index 9f64393..ee34666 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -404,6 +404,7 @@
         "_FILE_OFFSET_BITS_test.cpp",
         "float_test.cpp",
         "fnmatch_test.cpp",
+        "fts_test.cpp",
         "ftw_test.cpp",
         "getauxval_test.cpp",
         "getcwd_test.cpp",