Add missing prototype for p_realpath().

There is currently a small issue which this PR addresses:
- warning: implicit declaration of function 'p_realpath'
since p_realpath returns a char *, combined with the fact that GCC will assume an int is returned, bad things will happen. So provide a prototype for p_realpath() on OpenBSD.

This patch will also be also be included in the OpenBSD port of libgit2.