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 <[e-mail]>
Reviewed-by: Andreas Larsson <[e-mail]>
Signed-off-by: Andreas Larsson <[e-mail]>
Link: https://lore.kernel.org/r/57bc3b4[e-mail]

drivers/sbus/char/bbc_i2c.c

···
 	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

···
 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 */