[DF] Add RNTuple as a possible dependency

Necessary for Snapshot-related tests, now that snapshotting to RNTuple
is being added (root-project/root#15750).

Author: enirolf

root/dataframe/CMakeLists.txt

@@ -11,6 +11,10 @@ endif()
 
 set(DFLIBRARIES Core RIO Hist Tree MathCore TreePlayer ROOTDataFrame ROOTVecOps)
 
+if(ROOT_root7_FOUND)
+  list(APPEND DFLIBRARIES ROOTNTuple)
+endif()
+
 # ROOT-9975
 ROOTTEST_ADD_TEST(test_ROOT9975
                   COPY_TO_BUILDDIR ROOT9975.root