RCSwitch on Attiny85
The issue you face is that RCSwitch does not seem to support receive mode for ATtiny.

This is what you can find in RCSwitch.h:

// At least for the ATTiny X4/X5, receiving has to be disabled due to
// missing libm depencies (udivmodhi4)
#if defined( __AVR_ATtinyX5__ ) or defined ( __AVR_ATtinyX4__ )
#define RCSwitchDisableReceiving
#endif


Then all methods related to receptions are not defined in RCSwitch class for ATtiny MCUs:

#if not defined( RCSwitchDisableReceiving )
void enableReceive(int interrupt);
void enableReceive();
void disableReceive();
bool available();
void resetAvailable();

unsigned long getReceivedValue();
unsigned int getReceivedBitlength();
unsigned int getReceivedDelay();
unsigned int getReceivedProtocol();
unsigned int* getReceivedRawdata();
#endif


I am really not sure if there is a way to work around it, I have not looked at the code in RCSwitch.cpp. 

Maybe you best option would be to find another RC library.

For instance, if you can use Infra-Red RC, then there are libraries out there that could work for ATtiny:

https://github.com/shirriff/Arduino-IRremote