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: 2959a5f7 ("mm: add arch-independent testcases for RODATA")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lkml.kernel.org/r/20200819080026.918134-1-leon@kernel.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rodata_test.c

@@ -7,6 +7,7 @@
  */
 #define pr_fmt(fmt) "rodata_test: " fmt
 
+#include <linux/rodata_test.h>
 #include <linux/uaccess.h>
 #include <asm/sections.h>