syslog-ng: disable mongodb

syslog-ng: disable mongodb

libbson is a dependency of the mongo-c-driver that syslog-ng uses. Buildroot
doesn't package mongo-c-driver so syslog-ng uses the bundled one. The bundled
mongo-c-driver in turn may optionally use a bundled libbson. When Buildroot
builds libbson mongo-c-driver detects that and does not configure its bundled
libbson. This breaks the build of the syslog-ng mongodb module because it adds
the bundled libbson to the headers search path.

Disable the mongodb module to avoid this issue.

Fixes:
http://autobuild.buildroot.net/results/843/84331e9a168d8bdf2cceca8e9e1480611c1ecaed/
http://autobuild.buildroot.net/results/b5b/b5bad64abbf5764faf2a7129a1a25ad75c34980b/
http://autobuild.buildroot.net/results/3c3/3c32f4eb7436da06f3fb59f928363959df2a5e86/

Cc: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>