package/ripgrep: fix debug build

There is no --debug mode for cargo resulting in the following build
failure since the addition of this package with commit
4b0d1ef6:

error: Unknown flag: '--debug'

Fixes:
 - http://autobuild.buildroot.org/results/58e74bb056ec65680ecebaa559aa14bdebbf5c85
 - http://autobuild.buildroot.org/results/28c6364a89a6044d5a036614f7a6e59815efb770



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: keep the default 'dev' mode when in debug]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>