clang-tidy: filter out subproject headers

Subproject headers should be filtered out so that when building
outside of the CI docker container (or Yocto) we do not get tons
of clang-tidy failures for things present in those subprojects.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I35e909d667ce700e6c68ff3c2f269f1ef549dca3
diff --git a/.clang-tidy b/.clang-tidy
index b03b3ac..44ad8ad 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -339,7 +339,7 @@
 readability-uppercase-literal-suffix'
 
 WarningsAsErrors: '*'
-HeaderFilterRegex: '.*'
+HeaderFilterRegex: '(?!^subprojects).*'
 
 CheckOptions:
   - { key: readability-identifier-naming.ClassCase,          	value: CamelCase  }