[libc][NFC] Add const to RPC header members

Summary:
Make sure that these don't get modified.

Author: jhuber6

libc/shared/rpc.h

@@ -85,11 +85,11 @@ template <bool Invert> struct Process {
   RPC_INLINE Process &operator=(Process &&) = default;
   RPC_INLINE ~Process() = default;
 
-  uint32_t port_count = 0;
-  uint32_t *inbox = nullptr;
-  uint32_t *outbox = nullptr;
-  Header *header = nullptr;
-  Buffer *packet = nullptr;
+  const uint32_t port_count = 0;
+  const uint32_t *const inbox = nullptr;
+  uint32_t *const outbox = nullptr;
+  Header *const header = nullptr;
+  Buffer *const packet = nullptr;
 
   static constexpr uint64_t NUM_BITS_IN_WORD = sizeof(uint32_t) * 8;
   uint32_t lock[MAX_PORT_COUNT / NUM_BITS_IN_WORD] = {0};