mm/rodata_test.c: fix missing function declaration

The compilation with CONFIG_DEBUG_RODATA_TEST set produces the following
warning due to the missing include.

mm/rodata_test.c:15:6: warning: no previous prototype for 'rodata_test' [-Wmissing-prototypes]
15 | void rodata_test(void)
| ^~~~~~~~~~~

Fixes: 2959a5f726f6 ("mm: add arch-independent testcases for RODATA")
Signed-off-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Reviewed-by: Anshuman Khandual <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

mm/rodata_test.c

···
  */
 #define pr_fmt(fmt) "rodata_test: " fmt
 
+#include <linux/rodata_test.h>
 #include <linux/uaccess.h>
 #include <asm/sections.h>