nvramtool: Close file after use

mmap builds a new reference to the file, so the file
descriptor isn't necessary anymore. Close it.

Change-Id: I639fd13ff8f13cbdfce1d199d75744e56f2b19b3
Found-by: Coverity Scan
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/6475
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/nvramtool/cbfs.c

@@ -144,6 +144,7 @@ void open_cbfs(const char *filename)
 	}
 	cbfs_mapped = mmap(NULL, cbfs_stat.st_size, PROT_READ | PROT_WRITE,
 			MAP_SHARED, cbfs_fd, 0);
+	close(cbfs_fd);
 	if (cbfs_mapped == MAP_FAILED) {
 		printf("Couldn't map '%s'\n", filename);
 		exit(-1);