x86/shadow: Fix build with no PG_log_dirty

Gitlab Randconfig found:

  arch/x86/mm/shadow/common.c: In function 'shadow_prealloc':
  arch/x86/mm/shadow/common.c:1023:18: error: implicit declaration of function
      'paging_logdirty_levels'; did you mean 'paging_log_dirty_init'? [-Werror=implicit-function-declaration]
   1023 |         count += paging_logdirty_levels();
        |                  ^~~~~~~~~~~~~~~~~~~~~~
        |                  paging_log_dirty_init
  arch/x86/mm/shadow/common.c:1023:18: error: nested extern declaration of 'paging_logdirty_levels' [-Werror=nested-externs]

The '#if PG_log_dirty' expression is currently SHADOW_PAGING && !HVM &&
PV_SHIM_EXCLUSIVE.  Move the declaration outside.

Fixes: 33fb3a6612 ("x86/shadow: account for log-dirty mode when pre-allocating")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/include/asm/paging.h

@@ -142,6 +142,10 @@ struct paging_mode {
 /*****************************************************************************
  * Log dirty code */
 
+#define paging_logdirty_levels() \
+    (DIV_ROUND_UP(PADDR_BITS - PAGE_SHIFT - (PAGE_SHIFT + 3), \
+                  PAGE_SHIFT - ilog2(sizeof(mfn_t))) + 1)
+
 #if PG_log_dirty
 
 /* get the dirty bitmap for a specific range of pfns */
@@ -180,10 +184,6 @@ bool paging_mfn_is_dirty(const struct domain *d, mfn_t gmfn);
 #define L4_LOGDIRTY_IDX(pfn) ((pfn_x(pfn) >> (PAGE_SHIFT + 3 + PAGETABLE_ORDER * 2)) & \
                               (LOGDIRTY_NODE_ENTRIES-1))
 
-#define paging_logdirty_levels() \
-    (DIV_ROUND_UP(PADDR_BITS - PAGE_SHIFT - (PAGE_SHIFT + 3), \
-                  PAGE_SHIFT - ilog2(sizeof(mfn_t))) + 1)
-
 #ifdef CONFIG_HVM
 /* VRAM dirty tracking support */
 struct sh_dirty_vram {