Explicitly instantiate Set<ObjKey> in set.cpp

[tgoyne]

Set<ObjKey> is the single type not implicitly instantiated in Obj::get_setbase_ptr(), so it needs to be explicitly instantiated. Fixes the linker error seen in https://ci.realm.io/job/objc_pr/target=osx,xcode_version=14.3.1/10813/consoleFull. I have no idea why this works with LTO off but breaks with LTO on.

[coveralls-official]

Pull Request Test Coverage Report for Build github_pull_request_283538

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 96 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.005%) to 91.659%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/collection.hpp	1	91.05%
src/realm/index_string.cpp	1	87.81%
src/realm/sync/noinst/client_impl_base.hpp	1	93.91%
test/test_dictionary.cpp	1	99.85%
src/realm/array_string.cpp	2	88.0%
src/realm/object-store/shared_realm.cpp	2	93.74%
src/realm/table_view.cpp	2	94.18%
src/realm/util/file.cpp	2	81.73%
src/realm/sync/noinst/protocol_codec.hpp	3	76.72%
src/realm/util/assert.hpp	4	87.1%

Totals
Change from base Build 1819:	-0.005%
Covered Lines:	230790
Relevant Lines:	251793

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💛 - Coveralls