mm/early_ioremap: declare early_memremap_pgprot_adjust()

The mm/ directory can almost fully be built with W=1, which would help
in local development.  One remaining issue is missing prototype for
early_memremap_pgprot_adjust().

Thus add a declaration for this function.  Use mm/internal.h instead of
asm/early_ioremap.h to avoid missing type definitions and unnecessary
exposure.

Link: https://lkml.kernel.org/r/20220314165724.16071-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/early_ioremap.c b/mm/early_ioremap.c
index 74984c2..9bc12e5 100644
--- a/mm/early_ioremap.c
+++ b/mm/early_ioremap.c
@@ -17,6 +17,7 @@
 #include <linux/vmalloc.h>
 #include <asm/fixmap.h>
 #include <asm/early_ioremap.h>
+#include "internal.h"
 
 #ifdef CONFIG_MMU
 static int early_ioremap_debug __initdata;