fix: clippy error: returning the result of a `let` binding from a block

[haraldh]

error: returning the result of a `let` binding from a block
   --> src/syscall/mod.rs:305:9
    |
175 | /         let ret = match nr as _ {
176 | |             // MemorySyscallHandler
177 | |             libc::SYS_brk => self.brk(a.into()),
178 | |             libc::SYS_mmap => self.mmap(
...   |
302 | |             }
303 | |         };
    | |__________- unnecessary `let` binding
304 |
305 |           ret
    |           ^^^
    |