freedesktop_generate.sh: Don't hardcode ninja

When building shared-mime-info with samurai (https://github.com/michaelforney/samurai) rather than ninja it will fail because ninja is hardcoded in data/freedesktop_generate.sh. This can be solved by allowing the NINJA environment variable to be set so that the user can set NINJA=samu and it will work as expected. If the variable is not set it will default to ninja.

At least one other project that also uses the NINJA environment variable is antares (https://github.com/arescentral/antares) which has a custom python based build system.