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

arch/sparc/kernel/btext.c

@@ -137,7 +137,7 @@ static void scrollscreen(void)
 }
 #endif /* ndef NO_SCROLL */
 
-void btext_drawchar(char c)
+static void btext_drawchar(char c)
 {
 	int cline = 0;
 #ifdef NO_SCROLL