wrong usage of extern declaration in C files, and header cleanup

Thomas Laurentrequested to merge
fix-bad-extern-declaration-in-c-not-h-files into develop

wrong usage of extern declaration in C files, instead of headers, and fix hidden bugs by this wrong extern declaration duplication.

this fixes pure bugs because we cast variables to the wrong types, even if the bug has no consequence

it also separates 4G/5G include files, as the error is also in the headers mixup. Clean up header includes in F1AP.

Further remove some "CU/DUuniqinstances".

Edited by Robert Schmidt

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