Remove duplicated -Werror in dtc Makefile

The "-Werror" compiler flag is currently declared twice in the
Makefile, one time in WARNINGS, and one time in CFLAGS. Let's
remove one of them.

Signed-off-by: Thomas Huth <thuth@redhat.com>
[Moved remaining -Werror from WARNINGS to CFLAGS --dwg]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>