Examples make/compile fix

[MortenMiddelthon]

For raspberry pi library examples, added missing header file for sleep(), unistd.h.
Also changed pingpair_dyn.cpp to define variable "len" to 0 before trying to read from function. This would otherwise give me a compiler warning:

pingpair_dyn.cpp:172:42: warning: ‘len’ may be used uninitialized in this function [-Wmaybe-uninitialized]
radio.write( receive_payload, len );
^

[IANQIN99]

change sleep() to delay(milliseconds)
change len to sizeof(receive_payload)
you should get the file compiles=d
had encountered the same issues also