regulator: core: Don't use "proxy" headers

Update header inclusions to follow IWYU (Include What You Use)
principle.

Note that kernel.h is discouraged to be included as it's written
at the top of that file.

Signed-off-by: Andy Shevchenko <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

include/linux/regulator/coupler.h

···
 #ifndef __LINUX_REGULATOR_COUPLER_H_
 #define __LINUX_REGULATOR_COUPLER_H_
 
-#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/types.h>
 #include <linux/suspend.h>
 
 struct regulator_coupler;