Fix Due support

[per1234]

https://github.com/pmjdebruijn/Arduino-Entropy-Library/commit/aadd30945e98a96d171a554ddf7708998da37782 broke support for the Arduino Due by removing the #include directive for Arduino.h:

E:\arduino\libraries\Arduino-Entropy-Library\Entropy.cpp: In member function 'void EntropyClass::initialize()':

E:\arduino\libraries\Arduino-Entropy-Library\Entropy.cpp:62:32: error: 'pmc_enable_periph_clk' was not declared in this scope

   pmc_enable_periph_clk(ID_TRNG);

In the spirit of that commit, I have added the #include directive for the specific header file that contains the declaration of pmc_enable_periph_clk.