sparc64: fix sparse warning in btext.c

Fix following warning:
btext.c:140:6: warning: symbol 'btext_drawchar' was not declared. Should it be static?

Define the function static as it is only used in this file.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>