clang-tidy: Enable readability checks on repo

The readability-* checks in Clang-Tidy focus on improving code
readability by enforcing guidelines and suggesting improvements
that can enhance the clarity and maintainability of C++ code.
This enables readability-const-return-type check.

Change-Id: I4737f07188f6a173678dbe0192914dd5df0dd338
Signed-off-by: Pavithra Barithaya <pavithrabarithaya07@gmail.com>
diff --git a/systemd_target_signal.cpp b/systemd_target_signal.cpp
index 30e5d5b..7698c0e 100644
--- a/systemd_target_signal.cpp
+++ b/systemd_target_signal.cpp
@@ -75,8 +75,8 @@
     }
 }
 
-const std::string SystemdTargetLogging::processError(const std::string& unit,
-                                                     const std::string& result)
+std::string SystemdTargetLogging::processError(const std::string& unit,
+                                               const std::string& result)
 {
     auto targetEntry = this->targetData.find(unit);
     if (targetEntry != this->targetData.end())