Make stm32_interrupt_enable() and stm32_interrupt_disable() callable from C

[terrillmoore]

It appears that the intent was that stm32_interrupt_enable() and stm32_interrupt_disable() should be callable from C. But the header file (cores/arduino/stm32/interrupt.h) neglected to declare them extern "C", so they can't be. (There's an overload from stm32_interrupt_enable() using a std::function<> wrapper, and the header file makes that invisible when included in a .c file. But since the visible stm32_interrupt_enable() is implemented in a C++ file, you get a link error when you try to link. Ditto for stm32_interrupt_disable().

The solution is to wrap in the C-ish things in extern "C" in interrupt.h when compiling in C++; that way, the functions will be callable from C.

[dhineshkumarmcci]

@terrillmoore have tested with these changes. application works good but that requires changes in Sigfox library where interrupt_enable function is being called.

https://github.com/mcci-catena/MCCI_Sigfox_Image/blob/5f794cb04a1e47553b5d88410813491d41fde0ef/src/arduino_wrapper.c#L518-L526

I will do the necessary changes and push them to the Sigfox lib.

Also for PR #187 I will include the changes of branch issue188.

[terrillmoore]

requires changes in Sigfox library where interrupt_enable function is being called.

That's as expected. Thanks!