clang-tidy: Enable readability-string-compare check

This check finds string comparisons using the compare method.
A common mistake is to use the string’s compare method instead of
using the equality or inequality operators.

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I9d4b8aad27fa34345d08d2ba326b8a31cd3d99c6
diff --git a/.clang-tidy b/.clang-tidy
index 2c7ef0a..b3cb96d 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -250,6 +250,7 @@
 readability-simplify-subscript-expr,
 readability-static-accessed-through-instance,
 readability-static-definition-in-anonymous-namespace,
+readability-string-compare,
 readability-suspicious-call-argument,
 readability-uniqueptr-delete-release,
 readability-uppercase-literal-suffix'