build: lint header files with clang-tidy (#7527)

* build: clang-tidy headers when building libtransmission

* chore: revert `= default` workaround

It was introduced in 6909ec0bad to fix build issues with macOS 10.14. We
no longer support that version.

* fix: clang-tidy warnings for libtransmission

* build: clang-tidy headers when building tests

* fix: clang-tidy warnings for tests

* build: clang-tidy headers when building qt

* code review: don't manually edit mime-types.h

* code review: unify variable naming for static private members

libtransmission/tr-buffer.h

@@ -255,6 +255,7 @@ class StackBuffer final
 {
 public:
     StackBuffer() = default;
+    ~StackBuffer() override = default;
     StackBuffer(StackBuffer&&) = delete;
     StackBuffer(StackBuffer const&) = delete;
     StackBuffer& operator=(StackBuffer&&) = delete;