rootfs.py: close kernel_abi_ver_file

* fixes:
  oe-core/meta/lib/oe/rootfs.py:331: ResourceWarning: unclosed file <_io.TextIOWrapper name='pkgdata/mach/kernel-depmod/kernel-abiversion' mode='r' encoding='UTF-8'>
  kernel_ver = open(kernel_abi_ver_file).read().strip(' \n')

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>