libcurl: disable curldebug

curldebug is a more advanced form of debugging for curl which audits
source code with the checksource.pl tool, and treats warnings as errors.
Normally users won't want/need this so disable it since it leads to
failed builds when debug info is enabled (which is what people normally
want).
When buildroot does --enable-debug curl inherently enables curldebug too.

Solves bug #8041.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>