devel/libinotify: Prepare for the native inotify impl

The idea behind the change is to leave the library as-is to allow old programs
linked to it to run, but prevent new programs from linking to it. This is
achieved by replacing libinotify sys/inotify.h with a native one and removing
LDFLAGS from the .pc file

Differential Revision: https://reviews.freebsd.org/D50736