gdbstub: replace call to offset() with direct addition.

Address::offset() function has been removed in 39b92fb.

Signed-off-by: Vasant Karasulli <vkarasulli@suse.de>

src/debug/gdbstub.rs

@@ -221,7 +221,7 @@ pub mod svsm_gdbstub {
             };
 
             let guard = PerCPUPageMappingGuard::create_4k(phys.page_align())?;
-            return unsafe { write_u8(guard.virt_addr().offset(phys.page_offset()), value) };
+            return unsafe { write_u8(guard.virt_addr() + phys.page_offset(), value) };
         }
     }