x86/paravirt: Silence unused native_pv_lock_init() function warning

The native_pv_lock_init() function is only used in SMP configurations
and declared in asm/qspinlock.h which is not used in UP kernels, but
the function is still defined for both, which causes a warning:

  arch/x86/kernel/paravirt.c:76:13: error: no previous prototype for 'native_pv_lock_init' [-Werror=missing-prototypes]

Move the declaration to asm/paravirt.h so it is visible even
with CONFIG_SMP but short-circuit the definition to turn it
into an empty function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230803082619.1369127-7-arnd@kernel.org