clang-tidy: Restrict analysis to libpldm headers

Meson doesn't regard subproject headers as system headers by default,
which means they trigger analysis by clang-tidy. That's not something we
care about and clang-tidy seems to dislike things that gtest and gmock
are doing, so ignore them.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Iaa904f0517f7102b88ce7ee230b8360e2cc1dfd4
diff --git a/.clang-tidy b/.clang-tidy
index bdb6214..001c35f 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -207,7 +207,7 @@
 '
 
 WarningsAsErrors: '*'
-HeaderFilterRegex: '.*'
+HeaderFilterRegex: '(libpldm)|(src)/'
 CheckOptions:
   - { key: readability-identifier-naming.VariableCase,  value: lower_case }
   - { key: readability-identifier-naming.FunctionCase,  value: lower_case }