build fix: added missing declaration of mhl_dir_plus_file (reported by andrew_b)

./configure --enable-samba && make
../vfs/libvfs-mc.a(smbfs.o): In function `smbfs_get_path':
/home/andrew/work.c/mc/mc.master/vfs/smbfs.c:1214: undefined reference to `mhl_str_dir_plus_file'
collect2: ld returned 1 exit status

Signed-off-by: Sergei Trofimovich <slyfox@inbox.ru>

vfs/smbfs.c

@@ -49,6 +49,8 @@
 
 #include <string.h>
 
+#include <mhl/string.h>
+
 #include "vfs.h"
 #include "vfs-impl.h"
 #include "smbfs.h"