qcacmn: Migrate to linux/sched/signal.h

Linux kernel version 4.11 moved several signal related functions out of
linux/sched.h into linux/sched/signal.h. Add conditional compilation to
include the correct file based on the version of the Linux kernel being
compiled against.

Change-Id: Ic50a8ec0a76178b5231388983a890bdfdc4615e2
CRs-Fixed: 2094019

qdf/linux/src/qdf_threads.c

@@ -35,7 +35,11 @@
 #include <qdf_types.h>
 #include <qdf_trace.h>
 #include <linux/jiffies.h>
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 11, 0)
 #include <linux/sched.h>
+#else
+#include <linux/sched/signal.h>
+#endif /* KERNEL_VERSION(4, 11, 0) */
 #include <linux/delay.h>
 #include <linux/interrupt.h>
 #include <linux/export.h>