[RF] Include gsl_integration.h in RooFitMore instead of copy-pasting

Include `gsl_integration.h` in RooFitMore instead of copy-pasting code
from gsl. The linking is done in the same was as in
`math/mathmore/CMakeLists.txt`.

This should add no additional dependency compilcations, because
RooFitMore already linked against MathMore, which requires GSL to be
present.

roofit/roofitmore/CMakeLists.txt

@@ -55,9 +55,14 @@ ROOT_STANDARD_LIBRARY_PACKAGE(RooFitMore
     RIO
     MathCore
     Foam
+  BUILTINS
+    GSL
   ${EXTRA_DICT_OPTS}
 )
 
+target_include_directories(RooFitMore SYSTEM PRIVATE ${GSL_INCLUDE_DIR})
+target_link_libraries(RooFitMore PRIVATE ${GSL_LIBRARIES})
+
 # For recent clang, this can facilitate auto-vectorisation.
 # In RooFit, the errno side effect is not needed, anyway:
 if(NOT MSVC)