qcacmn: Fix kernel 4.15 timer list dependencies

With the changes made in Ibe0f8adc4df7bb98aceb509d438e241fac507393 and
Ic4cef0d6301230197443d4d5247188f2af643674 there are compilation issues
with converged code.

Resolve the issues by creating ifdefs for the timer_list definitions
according to kernel version.

Change-Id: Id9c29413ca0d21533a0afae245595051fa3a400f
CRs-Fixed: 2390884

qdf/inc/qdf_timer.h

@@ -27,7 +27,11 @@
 #include <qdf_types.h>
 #include <i_qdf_timer.h>
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 15, 0)
 typedef struct __qdf_timer_t qdf_timer_t;
+#else
+typedef __qdf_timer_t qdf_timer_t;
+#endif
 
 /**
  * qdf_timer_init() - initialize a timer