sbus: Add prototype for bbc_envctrl_init and bbc_envctrl_cleanup to header

This fixes the following build warning:

	drivers/sbus/char/bbc_envctrl.c:566:5: error: no previous prototype for ‘bbc_envctrl_init’ [-Werror=missing-prototypes]
	  566 | int bbc_envctrl_init(struct bbc_i2c_bus *bp)
	      |     ^~~~~~~~~~~~~~~~
	drivers/sbus/char/bbc_envctrl.c:594:6: error: no previous prototype for ‘bbc_envctrl_cleanup’ [-Werror=missing-prototypes]
	  594 | void bbc_envctrl_cleanup(struct bbc_i2c_bus *bp)
	      |      ^~~~~~~~~~~~~~~~~~~

which in the presence of -Werror=missing-prototypes yields a compiler
error.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/57bc3b43b405431a486162fb8c2558072e976747.1709591118.git.u.kleine-koenig@pengutronix.de

drivers/sbus/char/bbc_i2c.c

@@ -358,9 +358,6 @@ static struct bbc_i2c_bus * attach_one_i2c(struct platform_device *op, int index
 	return NULL;
 }
 
-extern int bbc_envctrl_init(struct bbc_i2c_bus *bp);
-extern void bbc_envctrl_cleanup(struct bbc_i2c_bus *bp);
-
 static int bbc_i2c_probe(struct platform_device *op)
 {
 	struct bbc_i2c_bus *bp;

drivers/sbus/char/bbc_i2c.h

@@ -82,4 +82,7 @@ extern int bbc_i2c_readb(struct bbc_i2c_client *, unsigned char *byte, int off);
 extern int bbc_i2c_write_buf(struct bbc_i2c_client *, char *buf, int len, int off);
 extern int bbc_i2c_read_buf(struct bbc_i2c_client *, char *buf, int len, int off);
 
+extern int bbc_envctrl_init(struct bbc_i2c_bus *bp);
+extern void bbc_envctrl_cleanup(struct bbc_i2c_bus *bp);
+
 #endif /* _BBC_I2C_H */