clang-tidy: ignore issues in subproject headers

This repository cannot control the settings and strictness of
subprojects, so we need to disable clang-tidy on those.  Leverage
the HeaderFilter option in the .clang-tidy to ignore issues in header
files and leverage the meson `.clang-tidy-ignore` feature to ignore
issues in C++ parts.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I01ff31ec62b364c310fb0aa87dc05883f4be9318
diff --git a/subprojects/.clang-tidy-ignore b/subprojects/.clang-tidy-ignore
new file mode 100644
index 0000000..72e8ffc
--- /dev/null
+++ b/subprojects/.clang-tidy-ignore
@@ -0,0 +1 @@
+*