Multiple include of the single header version lead to the multiple defined symbol error on windows

[David Fiedler]

I had a hard time with this library after using the progress bar in a second file. The problem is on the following line:

indicators/single_include/indicators/indicators.hpp

Line 678 in 312c49b

 size_t terminal_width() { return terminal_size().second; } 

After adding the inline specifier, everything works fine.

[Pranav]

Thanks. Will add the inline specifier and update master