minor update to fix msbuild condition to correct syntax
diff --git a/src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets b/src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets
index b1030ba..ad6725f 100644
--- a/src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets
+++ b/src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets
@@ -137,7 +137,7 @@
         <ProtoRoot>%(RelativeDir)</ProtoRoot>
       </Protobuf_Compile>
       <!-- Remove files not for compile. -->
-      <Protobuf_Compile Remove="@(Protobuf_Compile)" Condition=" !%(ProtoCompile) " />
+      <Protobuf_Compile Remove="@(Protobuf_Compile)" Condition=" '%(ProtoCompile)' != 'true' " />
       <!-- Ensure invariant Source=%(Identity). -->
       <Protobuf_Compile>
         <Source>%(Identity)</Source>