libobs: Fix missing getline on FreeBSD

Add definition on FreeBSD to enable getline in stdio, as it is not
(yet ?) available by default. According to the manpage getline was a
GNU extension but was standardized in POSIX.1-2008.

libobs/obs-nix.c

@@ -16,6 +16,10 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 ******************************************************************************/
 
+#ifdef __FreeBSD__
+#define _WITH_GETLINE
+#endif
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>