[cmake,bugfix] Fixes requiredness of module

Previously, dependencies were not correctly searched for as
required packages. This patch fixes this.

CMakeLists.txt

@@ -13,7 +13,7 @@ if(NOT (dune-common_DIR OR dune-common_ROOT OR
 endif()
 
 #find dune-common and set the module path
-find_package(dune-common)
+find_package(dune-common REQUIRED)
 list(APPEND CMAKE_MODULE_PATH ${dune-common_MODULE_PATH}
   "${PROJECT_SOURCE_DIR}/cmake/modules")