Reorder LinkHeaders function to allow compiling on Clang

[simco50]

Fixes this error in Clang

AMD_FidelityFX/include\ffx_api/ffx_api.hpp(63,5): error: call to function 'LinkHeaders' that is neither visible in the template definition nor found by argument-dependent lookup
   63 |     LinkHeaders(second, rest...);
      |     ^
AMD_FidelityFX/include\ffx_api/ffx_api.hpp(63,5): note: in instantiation of function template specialization 'ffx::LinkHeaders<ffxApiHeader, ffxApiHeader>' requested here
AMD_FidelityFX/include\ffx_api/ffx_api.hpp(84,19): note: in instantiation of function template specialization 'ffx::LinkHeaders<ffxApiHeader, ffxApiHeader, ffxApiHeader>' requested here
   84 |     auto header = LinkHeaders(desc.header...);
      |                   ^
AMD_FidelityFX/include\ffx_api/ffx_api.hpp(76,9): note: 'LinkHeaders' should be declared prior to the call site or in the global namespace
   76 | Header* LinkHeaders(Header& hdr)
      |