Fix fallout on MinGW64 from the previous commit.

Fix fallout on MinGW64 from the previous commit.

 src/systime.h (Time): Define as size_t, to be consistent with 64-bit
 Windows builds, where 'long' is a 32-bit type.
 src/w32inevt.h (w32_console_mouse_position): Update the argument
 types to use 'Time'.
 src/w32term.c (w32_mouse_position)
 (x_horizontal_scroll_bar_report_motion)
 (x_scroll_bar_report_motion): Update the argument types to use
 'Time'.