[Cmake][bugfix] Fix error in making headercheck configurable

The property must exist in order to have the exclusion macros work.
I only tested dune common which has no exclusions...

Thanks to Andreas Buhr for noticing!

cmake/modules/Headercheck.cmake

@@ -7,6 +7,8 @@ macro(setup_headercheck)
     # strip hidden files
     string(REGEX REPLACE "[^;]*/\\.[^;]*\\.hh;?" "" headers "${all_headers}")
     set_property(GLOBAL PROPERTY headercheck_list ${headers})
+  else()
+    set_property(GLOBAL PROPERTY headercheck_list "")
   endif()
 
   #define headercheck target