txr: fix musl build

srcpkgs/txr/patches/musl-fd_set.patch

@@ -0,0 +1,10 @@
+--- socket.c.orig	2016-03-21 10:54:59.650592629 +0100
++++ socket.c	2016-03-21 10:55:14.760403731 +0100
+@@ -24,6 +24,7 @@
+  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+  */
+ 
++#include <sys/select.h>
+ #include <stddef.h>
+ #include <stdio.h>
+ #include <stdlib.h>