NetBSD: enable detection of posix_fallocate()

posix_fallocate() is declared in unistd.h on NetBSD. Add the include
for proper detection on NetBSD 7.0 and up.

See http://netbsd.gw.com/cgi-bin/man-cgi?posix_fallocate++NetBSD-current

As that is violating the POSIX standards, a PR was opened at NetBSD
under http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=51287
for further tracking of the issue.

Change-Id: I40ec320677eef37bbc39f58e0bbac34f8cf7b8da
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

config.tests/unix/posix_fallocate/posix_fallocate.cpp

@@ -32,6 +32,10 @@
 ****************************************************************************/
 
 #include <fcntl.h>
+// NetBSD 7 has posix_fallocate, but in unistd.h instead of fcntl.h
+#ifdef __NetBSD__
+# include <unistd.h>
+#endif
 
 int main(int, char **)
 {