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/.gitignore b/.gitignore
index d7a8fcb..cd1bc79 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 # subproject directories
 /subprojects/*
 !/subprojects/*.wrap
+!/subprojects/.clang-tidy-ignore
 
 # build directory
 build*/*