sh: Refactor header include path addition

Shorten the code. No functional change intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20230219141555.2308306-3-masahiroy@kernel.org
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>