Fix testing with nss-crypt.

A glibc master build with --enable-nss-crypt using the NSS
crypto libraries fails during make check with the following error:

<command-line>:0:0: error: "USE_CRYPT" redefined [-Werror]
<command-line>:0:0: note: this is the location of the previous
definition

This is caused by commit 36975e8e
by H.J. Lu which replaces all = with +=. The fix is to undefine
USE_CRYPT before defining it to zero.

Committed as an obvious fix. Fixes the build issue on x86_64 with
no regressions.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>