src/select.c: Don't #include <sys/select.h> if it doesn't exist

We have to #include <sys/select.h> inside the block protected by
HAVE_SYS_SELECT_H, otherwise it will fail on (embedded) builds where
that file doesn't exist, such as the arm-none-eabi target on Debian
stable and Ubuntu 16.04.

Change-Id: I82a2dcbc55b6ee0e914881c5e09b80506c5de4f2

src/select.c

@@ -24,7 +24,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdbool.h>
-#include <sys/select.h>
 
 #include <osmocom/core/select.h>
 #include <osmocom/core/linuxlist.h>
@@ -33,6 +32,7 @@
 #include "../config.h"
 
 #ifdef HAVE_SYS_SELECT_H
+#include <sys/select.h>
 
 /*! \addtogroup select
  *  @{