Don't include linux/ header in osdSockUnsentCount.

Per the manpage in the file's comment, SIOCOUTQ is equivalent to the
TIOCOUTQ ioctl, whose value can be obtained by including <sys/ioctl.h>,
which allows us to avoid including any system-specific headers. This is
desirable so that, on a musl system, it won't be necessary to install
kernel headers in order to build epics-base.

Since it's first been checked into Git, the Linux kernel has defined
SIOCOUTQ to be TIOCOUTQ [1].

From the three main libc options on Linux: glibc and uclibc use the
kernel headers, so both ioctls are available; and musl defines only
TIOCOUTQ in their own headers.

[1] 1da177e4c3

modules/libcom/src/osi/os/Linux/osdSockUnsentCount.c

@@ -4,7 +4,6 @@
 * in file LICENSE that is included with this distribution.
 \*************************************************************************/
 
-#include <linux/sockios.h>
 #define EPICS_PRIVATE_API
 #include "osiSock.h"
 
@@ -14,7 +13,7 @@
  */
 int epicsSocketUnsentCount(SOCKET sock) {
     int unsent;
-    if (ioctl(sock, SIOCOUTQ, &unsent) == 0)
+    if (ioctl(sock, TIOCOUTQ, &unsent) == 0)
         return unsent;
     return -1;
 }