Fix mismatched class/struct definition warnings [-Wmismatched-tags]

Fixes the following Clang 7 warnings:
```
core/object.cpp:44:1: warning: '_ObjectDebugLock' defined as a struct here but previously declared as a class [-Wmismatched-tags]
core/variant_call.cpp:43:1: warning: '_VariantCall' defined as a struct here but previously declared as a class [-Wmismatched-tags]
drivers/gles3/rasterizer_storage_gles3.h:765:2: warning: 'MultiMesh' defined as a struct here but previously declared as a class [-Wmismatched-tags]
editor/editor_node.h:794:1: warning: 'EditorProgress' defined as a struct here but previously declared as a class [-Wmismatched-tags]
modules/bullet/rigid_body_bullet.h:230:17: warning: class 'KinematicUtilities' was previously declared as a struct [-Wmismatched-tags]
modules/bullet/space_bullet.h:60:1: warning: class 'btSoftBodyWorldInfo' was previously declared as a struct [-Wmismatched-tags]
scene/resources/world_2d.cpp:40:1: warning: 'SpatialIndexer2D' defined as a struct here but previously declared as a class [-Wmismatched-tags]
scene/resources/world.cpp:39:1: warning: 'SpatialIndexer' defined as a struct here but previously declared as a class [-Wmismatched-tags]
servers/audio/reverb_sw.cpp:60:1: warning: 'ReverbParamsSW' defined as a struct here but previously declared as a class [-Wmismatched-tags]
thirdparty/bullet/BulletSoftBody/btSoftBody.h:43:1: warning: 'btSoftBodyWorldInfo' defined as a struct here but previously declared as a class [-Wmismatched-tags]
```

1 files changed, 1 insertions, 1 deletions

diff --git a/servers/audio/reverb_sw.h b/servers/audio/reverb_sw.h
index 26b3fa5166..01977d1f61 100644
--- a/servers/audio/reverb_sw.h
+++ b/servers/audio/reverb_sw.h
@@ -34,7 +34,7 @@
 #include "core/os/memory.h"
 #include "core/typedefs.h"
 
-class ReverbParamsSW;
+struct ReverbParamsSW;
 
 class ReverbSW {
 public: