don't include libdatachannel examples in tarball #2964

Job	Run time
build (asio-debugging=on picker-debugging=on)	22m 55s
build (extensions=off logging=off streaming=off super-seeding=off share-mode=off predictive-piece...	16m 54s
check headers	4m 26s
build (crypto=gcrypt)	18m 20s
build (mmap-disk-io=off)	20m 22s
clang-tidy	9m 58s
Fuzzers	16m 11s
pre-commit	51s
Simulations	19m 0s
build dist (ubuntu-20.04)	18m 15s
Tests (webtorrent=on address-sanitizer=norecover undefined-sanitizer=norecover crypto=openssl)	18m 49s
build dist (ubuntu-22.04)	16m 50s
Tests (toolset=clang logging=off address-sanitizer=norecover undefined-sanitizer=norecover)	12m 0s
Tests (thread-sanitizer=norecover crypto=openssl release debug-symbols=on cxxflags=-Wno-tsan)	18m 49s
Tests (crypto=gnutls)	12m 57s
Tests (deprecated-functions=off)	11m 58s
	3h 58m 35s

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