metag: cleanup metag_ksyms.c includes

Minimise metag_ksyms.c includes to directly include the <asm/*.h> files
that declare a particular symbol, and not include any unnecessary ones.
Signed-off-by: James Hogan <james.hogan@imgtec.com>

arch/metag/kernel/metag_ksyms.c

 #include <linux/export.h>
-#include <linux/linkage.h>
-#include <linux/string.h>
-#include <linux/mm.h>
-#include <linux/interrupt.h>
-#include <linux/hardirq.h>
 
-#include <asm/checksum.h>
+#include <asm/div64.h>
 #include <asm/ftrace.h>
+#include <asm/page.h>
+#include <asm/string.h>
 #include <asm/tbx.h>
 
 EXPORT_SYMBOL(clear_page);