install_pmbootstrap: install losetup

Add losetup, so "pmbootstrap shutdown" doesn't fail because it tries to
use busybox losetup with the --json parameter that it doesn't support:

	[11:29:44] % sudo losetup --json --list
	losetup: unrecognized option: json
	BusyBox v1.36.1 (2024-06-12 11:52:11 UTC) multi-call binary.
	Usage: losetup [-rP] [-o OFS] {-f|LOOPDEV} FILE: associate loop devices
		losetup -c LOOPDEV: reread file size
		losetup -d LOOPDEV: disassociate
		losetup -a: show status
		losetup -f: show next free loop device
		-o OFS	Start OFS bytes into FILE
		-P	Scan for partitions
		-r	Read-only
		-f	Show/use next free loop device

Currently running into this with bpo CI while working on
build.postmarketos.org#140.

install_pmbootstrap.sh

@@ -41,6 +41,7 @@ add_remote_origin_original() {
 depends="
 	coreutils
 	git
+	losetup
 	openssl
 	procps
 	python3