move the sysconf call behind the _POSIX_SYNCHRONIZED_IO guard there

it is used for the msync call

apt-pkg/contrib/mmap.cc

@@ -186,7 +186,6 @@ bool MMap::Sync(unsigned long Start,unsigned long Stop)
    if ((Flags & UnMapped) == UnMapped)
       return true;
 
-   unsigned long long PSize = sysconf(_SC_PAGESIZE);
    if ((Flags & ReadOnly) != ReadOnly)
    {
       if (SyncToFd != 0)
@@ -198,6 +197,7 @@ bool MMap::Sync(unsigned long Start,unsigned long Stop)
       else
       {
 #ifdef _POSIX_SYNCHRONIZED_IO
+	 unsigned long long const PSize = sysconf(_SC_PAGESIZE);
 	 if (msync((char *)Base+(unsigned long long)(Start/PSize)*PSize,Stop - Start,MS_SYNC) < 0)
 	    return _error->Errno("msync", _("Unable to synchronize mmap"));
 #endif