service: Mark variables as [[maybe_unused]] where applicable (#5318)

Quite a few service functions are stubbed but still pop all their
arguments, which can lead to unused variable warnings.

We can mark the unused arguments with [[maybe_unused]] to silence these
warnings until a full implementation of these functions are made.

src/core/hle/service/ps/ps_ps.cpp

@@ -42,8 +42,8 @@ constexpr std::array<u8, 10> KeyTypes{{
 
 void PS_PS::EncryptDecryptAes(Kernel::HLERequestContext& ctx) {
     IPC::RequestParser rp(ctx, 0x4, 8, 4);
-    u32 src_size = rp.Pop<u32>();
-    u32 dest_size = rp.Pop<u32>();
+    auto src_size = rp.Pop<u32>();
+    [[maybe_unused]] const auto dest_size = rp.Pop<u32>();
 
     using CryptoPP::AES;
     std::array<u8, AES::BLOCKSIZE> iv;