[!749] Also install AddQuadMathFlags.cmake.

Merge branch 'install-FindQuadMath.cmake' into 'master'

ref:core/dune-common In is used downstream and needs to be installed.
Otherwise cmake for e.g. dune-geometry fails.

See merge request [!749]

  [!749]: gitlab.dune-project.org/core/dune-common/merge_requests/749

cmake/modules/CMakeLists.txt

@@ -3,6 +3,7 @@ install(FILES
   AddMETISFlags.cmake
   AddParMETISFlags.cmake
   AddPTScotchFlags.cmake
+  AddQuadMathFlags.cmake
   AddSuiteSparseFlags.cmake
   AddUMFPackFlags.cmake
   AddVcFlags.cmake