citra_qt/main: make SPEED_LIMIT_STEP static constexpr

MSVC does not seem to like using constexpr values in a lambda that were declared outside of it.
Previously on MSVC build the hotkeys to inc-/decrease the speed limit were not working correctly because in the lambda the SPEED_LIMIT_STEP had garbage values.
After googling around a bit I found: https://github.com/codeplaysoftware/computecpp-sdk/issues/95 which seems to be a similar issue.
Trying the suggested fix to make the variable static constexpr also fixes the bug here.