main/linux-postmarketos-qcom-msm8974: add findutils build dependency (!1090)

Forces to use normal "find" instead of busybox find.
This fixes unrecognized -printf parameter error dutring build:

 ...
 GEN     usr/initramfs_data.cpio
find: unrecognized: -printf
BusyBox v1.31.1 () multi-call binary.

Usage: find [-HL] [PATH]... [OPTIONS] [ACTIONS]
 ...