clang-tidy: Enable readability-redundant-string-cstr check

This check finds unnecessary calls to std::string::c_str() and
std::string::data().

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I4ff9cd7f1595a1cd7c6dbab4c7a80d6f7cf05ca2
diff --git a/.clang-tidy b/.clang-tidy
index d0c304e..dba1d69 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -273,6 +273,7 @@
 readability-redundant-member-init,
 readability-redundant-preprocessor,
 readability-redundant-smartptr-get,
+readability-redundant-string-cstr,
 readability-redundant-string-init,
 readability-simplify-boolean-expr,
 readability-simplify-subscript-expr,