core/hw/rsa: Make GetSignature() a const member function

This doesn't directly modify instance state, so it can be made const.

src/core/hw/rsa/rsa.h

@@ -14,7 +14,7 @@ public:
     RsaSlot() = default;
     RsaSlot(std::vector<u8> exponent, std::vector<u8> modulus)
         : init(true), exponent(std::move(exponent)), modulus(std::move(modulus)) {}
-    std::vector<u8> GetSignature(const std::vector<u8>& message);
+    std::vector<u8> GetSignature(const std::vector<u8>& message) const;
 
     explicit operator bool() const {
         // TODO(B3N30): Maybe check if exponent and modulus are vailid