Add explicit abseil str_cat import to build with new abseil version.

GTXiLib.podspec

diff --git a/GTXiLib.podspec b/GTXiLib.podspec
index cf8dede..351ccf5 100644
--- a/GTXiLib.podspec
+++ b/GTXiLib.podspec
@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "GTXiLib"
-  s.version      = "5.1.2"
+  s.version      = "5.1.3"
   s.summary      = "iOS Accessibility testing library."
   s.description  = <<-DESC
   iOS Accessibility testing library that works with XCTest based frameworks.
@@ -9,7 +9,7 @@
   s.license      = "Apache License 2.0"
   s.author       = "j-sid"
   s.platform     = :ios
-  s.source       = { :git => "https://github.com/google/GTXiLib.git", :tag => "5.1.2" }
+  s.source       = { :git => "https://github.com/google/GTXiLib.git", :tag => "5.1.3" }
   s.ios.deployment_target = "9.0"
   # Manually specify modulemap to use a custom umbrella header. CocoaPods' default
   # umbrella header includes public headers from GTXOOPLib, which include C++ symbols.

OOPClasses/check.cc

diff --git a/OOPClasses/check.cc b/OOPClasses/check.cc
index 8868bb1..2f37807 100644
--- a/OOPClasses/check.cc
+++ b/OOPClasses/check.cc
@@ -21,6 +21,7 @@
 #include <utility>
 #include <vector>
 
+#include <abseil/absl/strings/str_cat.h>
 #include <abseil/absl/types/optional.h>
 #include "metadata_map.h"
 #include "result.pb.h"

OOPClasses/localized_strings_manager.cc

diff --git a/OOPClasses/localized_strings_manager.cc b/OOPClasses/localized_strings_manager.cc
index 291fd30..f1fb651 100644
--- a/OOPClasses/localized_strings_manager.cc
+++ b/OOPClasses/localized_strings_manager.cc
@@ -20,6 +20,7 @@
 #include <string>
 
 #include <abseil/absl/container/flat_hash_map.h>
+#include <abseil/absl/strings/str_cat.h>
 #include <abseil/absl/strings/str_replace.h>
 #include <abseil/absl/types/optional.h>
 #include "localized_string_ids.h"