Fixes for the build configuration of InDetDiMuonMonitoring.

Made the package depend on FFTW, to fulfil a runtime dependency
for RooFFTConvPdf, and also cleaned up the configuration a bit at
the same time.

InnerDetector/InDetMonitoring/InDetDiMuonMonitoring/CMakeLists.txt

-################################################################################
-# Package: InDetDiMuonMonitoring
-################################################################################
+# Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 
 # Declare the package name:
 atlas_subdir( InDetDiMuonMonitoring )
 
 # External dependencies:
-find_package( ROOT COMPONENTS Core Tree MathCore Hist RIO pthread MathMore Minuit Minuit2 Matrix Physics HistPainter Rint RooFitCore RooFit Graf Graf3d Gpad Html Postscript Gui GX11TTF GX11 )
-find_package( GeoModelCore )
+find_package( ROOT COMPONENTS Core Hist RooFitCore RooFit Gpad )
+find_package( FFTW ) # Needed for RooFFTConvPdf
 
 # Component(s) in the package:
 atlas_add_component( InDetDiMuonMonitoring
-                     src/*.cxx
-                     src/components/*.cxx
-                     INCLUDE_DIRS ${ROOT_INCLUDE_DIRS} ${GEOMODELCORE_INCLUDE_DIRS}
-                     LINK_LIBRARIES ${ROOT_LIBRARIES} ${GEOMODELCORE_LIBRARIES} AthenaMonitoringLib xAODMuon xAODTracking GaudiKernel CxxUtils StoreGateLib SGtests )
+   src/*.h src/*.cxx src/components/*.cxx
+   INCLUDE_DIRS ${ROOT_INCLUDE_DIRS}
+   LINK_LIBRARIES ${ROOT_LIBRARIES} AthenaMonitoringLib xAODMuon xAODTracking
+   GaudiKernel CxxUtils )
 
 # Install files from the package:
 atlas_install_joboptions( share/*.py )
-