[RF] Fix visibility of the res/ directories.

After removing some CMake header cheating code, the headers in res/
became invisible across RooFit packages. Here, they are added to the
interface of the respective RooFit targets, but only for the
BUILD_INTERFACE.
This is OK as long as these headers will only be included in .cxx of
RooFit.

Author: hageboeck

roofit/batchcompute/CMakeLists.txt

@@ -7,6 +7,8 @@ ROOT_LINKER_LIBRARY(RooBatchCompute
     Imt
 )
 
+target_include_directories(RooBatchCompute INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/res>)
+
 if(vdt OR builtin_vdt)
   target_link_libraries(RooBatchCompute PUBLIC VDT)
 endif()

roofit/common/CMakeLists.txt

@@ -12,3 +12,5 @@ ROOT_LINKER_LIBRARY(RooFitCommon
   DEPENDENCIES
     Core
 )
+
+target_include_directories(RooFitCommon PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/res/>)