multimedia/libva-intel-hybrid-driver: unbreak build with clang 16

In file included from intel_hybrid_hostvld_vp9.cpp:67:
./intel_hybrid_hostvld_vp9_engine.h:116:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register INT                     iNumBits);
    ^~~~~~~~~

Reported by:	pkg-fallout

multimedia/libva-intel-hybrid-driver/Makefile

@@ -25,6 +25,7 @@ LIB_DEPENDS=	libdrm_intel.so:graphics/libdrm \
 		libva.so:multimedia/libva
 
 USES=		autoreconf compiler:c11 gmake libtool pkgconfig
+USE_CXXSTD=	gnu++98
 USE_GITHUB=	yes
 GH_ACCOUNT=	intel
 GNU_CONFIGURE=	yes