Add enum underlying type

The forward declaration of InterpolationMode already included its
underlying type. This commit does the same for ResourceRetType and
ResourceDimension.

renderdoc/driver/shaders/dxbc/dx_debug.h

@@ -29,14 +29,14 @@
 
 namespace DXBC
 {
-enum ResourceRetType;
+enum ResourceRetType : uint8_t;
 enum class InterpolationMode : uint8_t;
 class DXBCContainer;
 };
 
 namespace DXBCBytecode
 {
-enum ResourceDimension;
+enum ResourceDimension : uint8_t;
 enum SamplerMode;
 };