platform/generic_sdl.c: Stop using external
platform/generic_sdl.c: Stop using external

The framebuffer is now passed as an argument to mg_platform_fb_flush() and is
no more used as an external.

Signed-off-by: Nikolay Dimitrov <dfc921bb2213e22cc8f30fd615839493d2074dee@mail.bg>

picmaster/mikrogui