Make "Reset Pitch" keybind consistent with initial starting pitch

[aco4]

By default, the keybind for "Reset Pitch" is Keypad 5. The default angle is 320°, as specified in display.h by INITIAL_STARTING_PITCH. However, keybind.cpp inconsistently sets the pitch to 340°. You can see this discrepancy in-game: (1) Start a skirmish game, and (2) press Keypad 5. The camera pitch lowers.

I modified keybind.cpp to use INITIAL_STARTING_PITCH instead of 340°.