Fix the following warnings:

lib/gui/2d-primitives.c:88:6: warning: no
previous prototype for ‘gu_draw_line’ [-Wmissing-prototypes]
 void gu_draw_line(struct screen *sc,
      ^~~~~~~~~~~~
lib/gui/2d-primitives.c:174:6: warning: no
previous prototype for ‘gu_draw_circle’ [-Wmissing-prototypes]
 void gu_draw_circle(struct screen *sc,
      ^~~~~~~~~~~~~~

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>