powerpc/fsl-soc: hide unused const variable

powerpc/fsl-soc: hide unused const variable

vmpic_msi_feature is only used conditionally, which triggers a rare
-Werror=unused-const-variable= warning with gcc:

arch/powerpc/sysdev/fsl_msi.c:567:37: error: 'vmpic_msi_feature' defined but not used [-Werror=unused-const-variable=]
  567 | static const struct fsl_msi_feature vmpic_msi_feature =

Hide this one in the same #ifdef as the reference so we can turn on
the warning by default.

Fixes: 305bcf26128e ("powerpc/fsl-soc: use CONFIG_EPAPR_PARAVIRT for hcalls")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240403080702.3509288-2-arnd@kernel.org