Use reinterpret_cast to cast __FlashStringHelper to const char*

[Lauszus]

The cast from __FlashStringHelper to const char* produced the following warning:

/Users/Lauszus/GitHub/Arduino/build/macosx/work/Arduino.app/Contents/Resources/Java/hardware/arduino/avr/cores/arduino/Print.cpp: In member function 'size_t Print::print(const __FlashStringHelper*)':
/Users/Lauszus/GitHub/Arduino/build/macosx/work/Arduino.app/Contents/Resources/Java/hardware/arduino/avr/cores/arduino/Print.cpp:44: warning: '__progmem__' attribute ignored

As the F() macro is defined this way:

Arduino/hardware/arduino/avr/cores/arduino/WString.h

Line 38 in c86eed9

#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))

. We should use reinterpret_cast to cast it back to a const char*.