Move constexpr to headers

Fixes stricter C++17 criteria causing build errors

Author: Dario Berzano

DataFormats/Detectors/TPC/include/DataFormatsTPC/TPCSectorHeader.h

@@ -23,7 +23,7 @@ namespace TPC
 /// TPC specific header to be transported on the header stack
 struct TPCSectorHeader : public o2::header::BaseHeader {
   // Required to do the lookup
-  static const o2::header::HeaderType sHeaderType;
+  constexpr static const o2::header::HeaderType sHeaderType = "TPCSectH";
   static const uint32_t sVersion = 1;
 
   TPCSectorHeader(int s)

DataFormats/Detectors/TPC/src/TPCSectorHeader.cxx

@@ -10,4 +10,4 @@
 
 #include "DataFormatsTPC/TPCSectorHeader.h"
 
-constexpr o2::header::HeaderType o2::TPC::TPCSectorHeader::sHeaderType = "TPCSectH";
+constexpr o2::header::HeaderType o2::TPC::TPCSectorHeader::sHeaderType;

Framework/Core/include/Framework/DataProcessingHeader.h

@@ -44,7 +44,7 @@ namespace framework
 struct DataProcessingHeader : public header::BaseHeader
 {
   // Required to do the lookup
-  static const o2::header::HeaderType sHeaderType;
+  constexpr static const o2::header::HeaderType sHeaderType = "DataFlow";
   static const uint32_t sVersion = 1;
 
   // allows DataHeader::SubSpecificationType to be used as generic type in the code

Framework/Core/src/DataProcessingHeader.cxx

@@ -16,7 +16,7 @@ namespace o2
 namespace framework
 {
 
-constexpr o2::header::HeaderType DataProcessingHeader::sHeaderType = "DataFlow";
+constexpr o2::header::HeaderType DataProcessingHeader::sHeaderType;
 
 } // namespace framework
 } // namespace o2