RHI: fix unresolved exports
Clang 13 needs RHI_API on the class template definitions to match the declarations. Unsure if this is a regression for Clang 13 or due to some other reason, but adding them doesn't negatively affect any other compile targets.
#rnx
#rb trivial
#preflight 61d4a9826511bc498e3cc6ec
#ROBOMERGE-AUTHOR: joe.kirchoff
#ROBOMERGE-SOURCE: CL 18512106 in //UE5/Release-5.0/... via CL 18512119
#ROBOMERGE-BOT: STARSHIP (Release-Engine-Staging -> Release-Engine-Test) (v899-18417669)
[CL 18512138 by joe kirchoff in ue5-release-engine-test branch]