Open source build fix for "[Xcode] Symlink to missing headers and che…

…ck in missing TBDs, instead of mutating the public SDK"

https://bugs.webkit.org/show_bug.cgi?id=259831
rdar://problem/113764577

Unreviewed build fix.

Re-apply https://commits.webkit.org/267370@main because it got undone in
a libwebrtc sync.

* Source/ThirdParty/libwebrtc/libwebrtc.xcodeproj/project.pbxproj:

Originally-landed-as: 267370@main (4ee5a0c). https://bugs.webkit.org/show_bug.cgi?id=266423
Canonical link: https://commits.webkit.org/272063@main

Source/ThirdParty/libwebrtc/libwebrtc.xcodeproj/project.pbxproj

@@ -23616,6 +23616,7 @@
 				"$(PROJECT_DIR)/Configurations/libwebrtc.exp",
 				"$(PROJECT_DIR)/Configurations/libwebrtc.testing.exp",
 				"$(PROJECT_DIR)/Configurations/libwebrtc.debug.exp",
+				"$(WK_DERIVED_SDK_HEADERS_DIR)",
 			);
 			name = "Generate Export Files";
 			outputFileListPaths = (