fdisk: document FDISK_DEBUG

Add the corresponding entry in the manpage.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>

fdisks/fdisk.8

@@ -271,6 +271,11 @@ The option `dump partition table to file' is missing.
 .\" Jakub Jelinek (jj@sunsite.mff.cuni.cz)
 .\" Andreas Neuper (ANeuper@GUUG.de)
 .\" and many others.
+
+.SH ENVIRONMENT
+.IP FDISK_DEBUG=0xffff
+enables debug output
+
 .SH "SEE ALSO"
 .BR cfdisk (8),
 .BR sfdisk (8),