Correctly mark protobuf as required in find_package. (#17021)

At the point at which we call find_package() to locate protobuf, we know
that we do, in fact, need it for this build. Given this, we should be
marking it as required in the find_package() call so that if it’s not
found the configuration phase fails instead of things breaking in
potentially strange ways later on during the build itself.

CMakeLists.txt

@@ -555,7 +555,7 @@ if(ENABLE_ACLK OR ENABLE_EXPORTER_PROMETHEUS_REMOTE_WRITE)
                         set(Protobuf_USE_STATIC_LIBS On)
                 endif()
 
-                find_package(Protobuf)
+                find_package(Protobuf REQUIRED)
         endif()
 
         set(ENABLE_PROTOBUF True)