m68k: atari: Make atari_stram_map_pages() static

When building with W=1:

    arch/m68k/atari/stram.c:118:12: warning: no previous prototype for ‘atari_stram_map_pages’ [-Wmissing-prototypes]
      118 | int __init atari_stram_map_pages(void)
	  |            ^~~~~~~~~~~~~~~~~~~~~

Fix this by making atari_stram_map_pages() static.
There was never a user outside arch/m68k/atari/stram.c.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/fa86b6d7e54d96ac52f574ed00f96e339f46990f.1694613528.git.geert@linux-m68k.org

arch/m68k/atari/stram.c

@@ -115,7 +115,7 @@ void __init atari_stram_reserve_pages(void *start_mem)
  * This function is called as arch initcall to reserve the pages needed for
  * ST-RAM management, if the kernel does not reside in ST-RAM.
  */
-int __init atari_stram_map_pages(void)
+static int __init atari_stram_map_pages(void)
 {
 	if (!kernel_in_stram) {
 		/*