xen/riscv: fix build issue for bullseye-riscv64 container

Address compilation error on bullseye-riscv64 container:
   undefined reference to `guest_physmap_remove_page`

Since there is no current implementation of `guest_physmap_remove_page()`,
a stub function has been added.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/riscv/stubs.c

@@ -381,6 +381,12 @@ int arch_get_paging_mempool_size(struct domain *d, uint64_t *size)
     BUG_ON("unimplemented");
 }
 
+int guest_physmap_remove_page(struct domain *d, gfn_t gfn, mfn_t mfn,
+                              unsigned int page_order)
+{
+    BUG_ON("unimplemented");
+}
+
 /* delay.c */
 
 void udelay(unsigned long usecs)