clang-tidy: Enable readability-make-member-function-const check

Finds non-static member functions that can be made const because the
functions don’t use this in a non-const way.

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: Iff3a367401d571f9fcff90539aaab59e2c66e627
diff --git a/.clang-tidy b/.clang-tidy
index 53684d8..0b9563b 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -235,6 +235,7 @@
 readability-function-size,
 readability-identifier-naming,
 readability-isolate-declaration,
+readability-make-member-function-const,
 readability-misleading-indentation,
 readability-misplaced-array-index,
 readability-named-parameter,