[ELF] - Add virtual destructor to LinkerScriptBase class. NFC.

Absence of it caused a clang warning:
warning: 'lld::elf::LinkerScriptBase' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]

At fact we don't need it here because do not destroy this object by
base pointer.

llvm-svn: 280916