[lldb] Declare extern template instantiation to fix linking issues.

NativeProcessELF::GetELFImageInfoAddress<...>() is declared in NativeProcessELF.h, but only defined in NativeProcessELF.cpp. Via some optimized builds (e.g. thinlto), this instantiation may be removed when it is used in a different TU (NativeProcessELFTest.cpp).