doc: Only install if BUILD_SPHINX_HTML==YES
doc: Only install if BUILD_SPHINX_HTML==YES

Fixes GHC #11818.

(cherry picked from commit c6d6a18d85e5e2d9bb5904e6919e8a8d7e31c4c5)

haskell/haddock,idontgetoutmuch/haddock,niteria/haddock,Fuuzetsu/haddock,idontgetoutmuch/haddock,haskell/haddock,Fuuzetsu/haddock,haskell/haddock,Fuuzetsu/haddock,Helkafen/haddock,Helkafen/haddock,idontgetoutmuch/haddock,niteria/haddock,Fuuzetsu/haddock,Helkafen/haddock,niteria/haddock,haskell/haddock,haskell/haddock