gpt: Correct naming. efi_* also conflicts with some efi headers

Change-Id: I19b5b2c4609da8474b26588ae8d7d4caf72a826b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2018
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>