bigfile/virtmem: Forward-decl bitmap

Else it won't compile with C++. This needs my patch to CCAN to work.
Patch description and problem details are here:

http://git.ozlabs.org/?p=ccan;a=commitdiff;h=39b46a02b719

ccan @ 39b46a02

-Subproject commit ce4660aa49404530a608a37754fae16fbb3fde02
+Subproject commit 39b46a02b7199786e926dfcb4086544789e30a68

bigfile/virtmem.c

@@ -30,6 +30,7 @@
 #include <wendelin/bug.h>
 
 #include <ccan/minmax/minmax.h>
+#include <ccan/bitmap/bitmap.h>
 
 #include <sys/mman.h>
 #include <errno.h>

include/wendelin/bigfile/virtmem.h

@@ -35,12 +35,13 @@
 #include <wendelin/list.h>
 #include <wendelin/bigfile/types.h>
 #include <wendelin/bigfile/pagemap.h>
-#include <ccan/bitmap/bitmap.h> // XXX can't forward-decl for bitmap
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+typedef struct bitmap bitmap;
+
 typedef struct RAM RAM;
 typedef struct RAMH RAMH;
 typedef struct Page Page;