lib: fix implicit users of kernel.h for TAINT_WARN

A pending header cleanup will cause this to show up as:

lib/average.c:38: error: 'TAINT_WARN' undeclared (first use in this function)
lib/list_debug.c:24: error: 'TAINT_WARN' undeclared (first use in this function)

and TAINT_WARN comes from include/linux/kernel.h file.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

lib/average.c

@@ -7,6 +7,7 @@
 
 #include <linux/module.h>
 #include <linux/average.h>
+#include <linux/kernel.h>
 #include <linux/bug.h>
 #include <linux/log2.h>
 

lib/list_debug.c

@@ -8,6 +8,7 @@
 
 #include <linux/module.h>
 #include <linux/list.h>
+#include <linux/kernel.h>
 
 /*
  * Insert a new entry between two known consecutive entries.