Forward-declare pledge() and unveil() on OpenBSD

These are declared in unistd.h, but since _POSIX_SOURCE is defined
they are not visible. Conditionally duplicating the declarations here
seems less likely to bite me in the ass later on than making a bunch
of other BSD-specific things visible.

main.c

@@ -31,6 +31,11 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#ifdef __OpenBSD__
+int pledge(const char *, const char *);
+int unveil(const char *, const char *);
+#endif
+
 #define HOST "::1"
 #define PORT 7070