dmon_extra header cannot declare extra functions

[franko]

I noticed the dmon_extra.h wrap the declarations of the functions dmon_watch_add/rm with the directive #if DMON_OS_LINUX but this doesn't work because DMON_OS_LINUX is only defined when DMON_IMPL is activated so it doesn't work for simple declarations.

I propose to declare dmon_watch_add/rm unconditionally or otherwise ensure that DMON_OS_LINUX and the other variables are always initialized even outside of the implementation.

[septag]

thanks. fixed in the latest commit