vst3-host: fixture builds on Windows (bus flags are i32 there)

The VST3 bindings type C enum constants as i32 on Windows and u32
elsewhere, so assigning kDefaultActive to BusInfo.flags (u32) failed to
compile and seven integration tests could not build their fixture.
Cast it like the other constants in this file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/vst3-host/tests/fixture/src/lib.rs

@@ -52,7 +52,7 @@ fn audio_bus(bus: *mut BusInfo, dir: BusDirection, name: &str) {
     b.channelCount = 2;
     copy_c16(name, &mut b.name);
     b.busType = BusTypes_::kMain as BusType;
-    b.flags = BusInfo_::BusFlags_::kDefaultActive;
+    b.flags = BusInfo_::BusFlags_::kDefaultActive as u32;
 }
 
 unsafe fn channels<'a>(bus: &AudioBusBuffers, frames: usize) -> Vec<&'a mut [f32]> {
@@ -461,7 +461,7 @@ impl IComponentTrait for Synth {
             b.channelCount = 16;
             copy_c16("Notes", &mut b.name);
             b.busType = BusTypes_::kMain as BusType;
-            b.flags = BusInfo_::BusFlags_::kDefaultActive;
+            b.flags = BusInfo_::BusFlags_::kDefaultActive as u32;
             return kResultOk;
         }
         kInvalidArgument