[casCap] side mode causes script error and instant end in SP

[NikkoJT]

When CasCap is enabled in Side mode, and the mission is started in SP, CasCap generates a zero divisor error and instantly ends the mission.

This is because CasCap uses playableUnits, which does not exist in SP. It can be fixed by also checking switchableUnits.

[SamLex]

Testing on dev (0903b21) - PASS

• Tested with BLUFOR 50% CasCap on v3.5.7 in SP - as reported, script error and instant game over
• Tested with BLUFOR 50% CasCap on dev in SP - no script error or instant game over. CasCap
  triggers correctly when 50% of "playable" BLUFOR units are killed
• Tested with BLUFOR 50% CasCap on dev in non-dedicated MP - no script error or instant game over. CasCap triggers correctly when 50% of "playable" BLUFOR units are killed