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]
```

2 files changed, 2 insertions, 2 deletions

diff --git a/modules/bullet/rigid_body_bullet.h b/modules/bullet/rigid_body_bullet.h
index cd2f215906..25dac30951 100644
--- a/modules/bullet/rigid_body_bullet.h
+++ b/modules/bullet/rigid_body_bullet.h
@@ -227,7 +227,7 @@ public:
 
 	void init_kinematic_utilities();
 	void destroy_kinematic_utilities();
-	_FORCE_INLINE_ class KinematicUtilities *get_kinematic_utilities() const { return kinematic_utilities; }
+	_FORCE_INLINE_ KinematicUtilities *get_kinematic_utilities() const { return kinematic_utilities; }
 
 	_FORCE_INLINE_ btRigidBody *get_bt_rigid_body() { return btBody; }
 
diff --git a/modules/bullet/space_bullet.h b/modules/bullet/space_bullet.h
index d815898ffd..c3d55cbbb1 100644
--- a/modules/bullet/space_bullet.h
+++ b/modules/bullet/space_bullet.h
@@ -57,7 +57,7 @@ class btDiscreteDynamicsWorld;
 class btEmptyShape;
 class btGhostPairCallback;
 class btSoftRigidDynamicsWorld;
-class btSoftBodyWorldInfo;
+struct btSoftBodyWorldInfo;
 class ConstraintBullet;
 class CollisionObjectBullet;
 class RigidBodyBullet;