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.

libwebrtc imports some SDK headers which are not available on all
platforms by default (like Metal), so it needs to depend on the
SDKAdditions directory being available.

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

Canonical link: https://commits.webkit.org/267370@main

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

@@ -22278,6 +22278,7 @@
 			inputPaths = (
 				"$(PROJECT_DIR)/Configurations/libwebrtc.exp",
 				"$(PROJECT_DIR)/Configurations/libwebrtc.testing.exp",
+				"$(WK_DERIVED_SDK_HEADERS_DIR)",
 			);
 			name = "Generate Export Files";
 			outputFileListPaths = (