install_pmbootstrap: add multipath-tools

pmbootstrap now depends on kpartx, which is provided by multipath-tools
in alpine.

Related: pmb MR 2450

install_pmbootstrap.sh

@@ -46,6 +46,7 @@ depends="
 	coreutils
 	git
 	losetup
+	multipath-tools
 	openssl
 	procps
 	python3