[ObjectYAML] - An attempt to fix BB after commit of D95140.

D95140 introduced `static constexpr StringRef TypeStr = "SectionHeaderTable";` member
of `SectionHeaderTable` with in-class initialized. BB reports the link error:

/usr/bin/ld: lib/libLLVMObjectYAML.a(ELFYAML.cpp.o): in function
`llvm::yaml::MappingTraits<std::unique_ptr<llvm::ELFYAML::Chunk, std::default_delete<llvm::ELFYAML::Chunk> > >::mapping(llvm::yaml::IO&, std::unique_ptr<llvm::ELFYAML::Chunk, std::default_delete<llvm::ELFYAML::Chunk> >&)':
ELFYAML.cpp:(.text._ZN4llvm4yaml13MappingTraitsISt10unique_ptrINS_7ELFYAML5ChunkESt14default_deleteIS4_EEE7mappingERNS0_2IOERS7_+0x58): undefined reference to `llvm::ELFYAML::SectionHeaderTable::TypeStr'
/usr/bin/ld: ELFYAML.cpp:(.text._ZN4llvm4yaml13MappingTraitsISt10unique_ptrINS_7ELFYAML5ChunkESt14default_deleteIS4_EEE7mappingERNS0_2IOERS7_+0x353):undefined reference to `llvm::ELFYAML::SectionHeaderTable::TypeStr'
/usr/bin/ld: ELFYAML.cpp:(.text._ZN4llvm4yaml13MappingTraitsISt10unique_ptrINS_7ELFYAML5ChunkESt14default_deleteIS4_EEE7mappingERNS0_2IOERS7_+0x6e5): undefined reference to `llvm::ELFYAML::SectionHeaderTable::TypeStr'

This patch adds a definition to cpp file, I guess it should fix the issue.