Add missing install headers

[ironage]

The validate-install-headers job on realm-core-stable has been failing since #7157

No changelog, since this hasn't been released yet.

[coveralls-official]

Pull Request Test Coverage Report for Build james.stone_429

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 82 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.007%) to 91.699%

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Files with Coverage Reduction	New Missed Lines	%
src/realm/index_string.cpp	1	87.81%
src/realm/array_string.cpp	2	88.0%
src/realm/object-store/sync/sync_manager.cpp	2	86.58%
src/realm/sync/noinst/server/server_history.cpp	2	67.9%
src/realm/util/file.cpp	2	81.73%
src/realm/uuid.cpp	2	97.06%
test/test_sync.cpp	2	94.14%
src/realm/sync/noinst/protocol_codec.hpp	3	76.21%
src/realm/alloc_slab.cpp	5	92.89%
src/realm/sync/network/network.hpp	7	85.41%

Totals
Change from base Build 1860:	0.007%
Covered Lines:	231728
Relevant Lines:	252706

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💛 - Coveralls

[kiburtse]

why don't we run this headers check on every pr for at least one builder? The check itself takes no time compared to compilation and especially baas tests...

[ironage]

@kiburtse I don't think we have anything to lose, I've added it to PR triggers in the latest commit.