package/chartjs: security bump to 2.9.4

package/chartjs: security bump to 2.9.4

CVE-2020-7746 (https://nvd.nist.gov/vuln/detail/CVE-2020-7746)

    The options parameter is not properly sanitized when it is processed.
    When the options are processed, the existing options (or the defaults
    options) are deeply merged with provided options. However, during this
    operation, the keys of the object being set are not checked, leading to
    a prototype pollution.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>