win32: only count subdirectories if necessary

Commit 7fb95a2a5 (win32: try to get link count for directories)
allowed stat(2) to report accurate values of st_nlink for
directories.

There are only a couple of places in busybox-w32 where these values
are required.  Disable counting of subdirectories by default and
only enable it when necessary.

Microsoft kindly provide directories to test edge cases like this:

   C:/Windows/WinSxS           (contains many subdirectories)
   C:/Windows/WinSxS/Manifests (contains many files)

Adds 84-112 bytes.

1 files changed, 1 insertions, 0 deletions

diff --git a/include/mingw.h b/include/mingw.h
index 4c3354864..6259bc2c6 100644
--- a/include/mingw.h
+++ b/include/mingw.h
@@ -349,6 +349,7 @@ struct mingw_stat {
 #define st_mtime st_mtim.tv_sec
 #define st_ctime st_ctim.tv_sec
 
+int count_subdirs(const char *pathname);
 int mingw_lstat(const char *file_name, struct mingw_stat *buf);
 int mingw_stat(const char *file_name, struct mingw_stat *buf);
 int mingw_fstat(int fd, struct mingw_stat *buf);