build: fix build on mingw with netcf available

The autobuilder pointed out an odd failure on mingw:
../../src/interface/netcf_driver.c:644:5: error: unknown field 'close_used_without_including_unistd_h' specified in initializer
cc1: warnings being treated as errors

This is because the gnulib headers #define close to different strings,
according to which headers are included, in order to work around some
odd mingw problems with close(), and these defines happen to also
affect field members declared with a name of struct foo.close. As long
as all headers are included before both the definition and use of the
struct, the various #define doesn't matter, but the netcf file hit
an instance where things were included in a different order.  Fix this
for all clients that use a struct member named 'close'.

* src/driver.h: Include <unistd.h> before using 'close'.

src/driver.h

@@ -8,6 +8,7 @@
 
 # include "config.h"
 
+# include <unistd.h>
 # include <libxml/uri.h>
 
 # include "internal.h"