Close #12826: fix socketmodule.c for OpenBSD, include sys/uio.h

Patch written by Remi Pointel.
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index 81d1ce1..5878ebb 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -156,6 +156,10 @@
 # undef HAVE_GETHOSTBYNAME_R_6_ARG
 #endif
 
+#if defined(__OpenBSD__)
+# include <sys/uio.h>
+#endif
+
 #ifndef WITH_THREAD
 # undef HAVE_GETHOSTBYNAME_R
 #endif