Use err() instead of error() in mkbootfs

error() is a glibc extension that is not available in musl libc.
err() is a BSD extension, but is present in glibc, musl libc and
bionic.

Test: m USE_HOST_MUSL=true host-native -k
Change-Id: I2ba7bddc2a725826be8be7f80208587b81de9238