Bump third_party/libwebsockets/repo from `ec6d5ac` to `18fdb0d` #5246

Job	Run time
REPL Tests - Linux	46m 56s
Test Suites - Darwin (no-ble-asan-clang)	1h 16m 9s
Test Suites - Darwin (no-ble-tsan-clang)	2h 3m 25s
Test Suites - Linux (no-ble-tsan-clang)	1h 34m 53s
REPL Tests - Darwin	0s
	5h 41m 23s

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