Fix and improve TPC Loopers implementation

[jackal1-66]

• Moved the TPCLoopers inclusion to Generator.cxx
• Made forward declaration for GenTPCLoopers
• Switched to normal pointer instead of unique_ptr (destroyed in Generator destructor) ==> moved outside ifdef scope, this was the main cause of the segmentation issue when using the external generators, such as Update tune from SCCR to QCDCR for pO and OO O2DPG#2256

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 315600e at 2026-02-04 15:13:

++ cp /sw/slc9_x86-64/O2/15013-slc9_x86-64-local1/compile_commands.json .
+++ python3 -c 'import json, os; print(os.path.commonpath([x["file"] for x in json.loads(open("compile_commands.json").read()) if "sw/BUILD" not in x["file"] and "G__" not in x["file"] and x["file"].endswith(".cxx")]))'
++ O2_SRC=/sw/SOURCES/O2/15013-slc9_x86-64/0
++ [[ -e /sw/SOURCES/O2/15013-slc9_x86-64/0/CMakeLists.txt ]]

Full log here.