x86: include proper prototypes for rodata_test

extern should not appear in C files.  Also, the definitions
do not match the prototype currently, not sure what way you
want to go with this, I've switched the prototype to return
int, but I can see going to the void return as well.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/test_rodata.c

@@ -10,8 +10,8 @@
  * of the License.
  */
 #include <linux/module.h>
+#include <asm/cacheflush.h>
 #include <asm/sections.h>
-extern int rodata_test_data;
 
 int rodata_test(void)
 {