i386: Fix the missing Rust HPET configuration option

The configuration option of Rust HPET is missing, so that PC machine
can't boot with "hpet=on" when QEMU Rust support is enabled.

Add the Rust HPET configuration option.

Fixes: d128c341a744 ("i386: enable rust hpet for pc when rust is enabled")
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250217154416.3144571-1-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/timer/Kconfig

@@ -1,2 +1,3 @@
 config X_HPET_RUST
     bool
+    default y if PC && HAVE_RUST