grc: function probe waits for flowgraph initialization

Wait until the flowgraph is done initializing before running the
probe loop. The real problem had to do with referencing a
variable that may not have been created yet, due to the ordering
of autogenerated code. Waiting until the flowgraph has been started
ensures that everything has been initialized.

Signed-off-by: Jeff Long <willcode4@gmail.com>

blocks/variable_function_probe.block.yml

@@ -33,6 +33,7 @@ templates:
     var_make: self.${id} = ${id} = ${value}
     make: |+
         def _${id}_probe():
+          self.flowgraph_started.wait()
           while True:
             <% obj = 'self' + ('.' + block_id if block_id else '') %>
             val = ${obj}.${function_name}(${function_args})