Adding cmake find_package(Threads) to (hopefully) bring in pthreads

Signed-off-by: AMZN-Gene <[email protected]>

package-system/AWSGameLiftServerSDK/test/CMakeLists.txt

@@ -40,6 +40,7 @@ endfunction()
 
 PROJECT(test_AWSGameLift VERSION 1.0 LANGUAGES CXX)
 
+find_package(Threads REQUIRED)
 find_package(OpenSSL)
 find_package(AWSGameLiftServerSDK)