apt-private: #ifdef HAVE_STRUCT_STATFS_F_TYPE, not #if

While autotools defines all macros to 1 explicitly, CMake only
defines them without a value. In such a case, #if fails with an
error and #ifdef works.

In preparation for a possible switch to CMake and to clean up
the code (rest uses #ifdef), use #ifdef here

apt-private/private-download.cc

@@ -143,7 +143,7 @@ bool CheckFreeSpaceBeforeDownload(std::string const &Dir, unsigned long long Fet
       {
 	 struct statfs Stat;
 	 if (statfs(Dir.c_str(),&Stat) != 0
-#if HAVE_STRUCT_STATFS_F_TYPE
+#ifdef HAVE_STRUCT_STATFS_F_TYPE
 	       || Stat.f_type != RAMFS_MAGIC
 #endif
 	    )