license_image.bbclass: close package.manifest file

license_image.bbclass: close package.manifest file

* fixes:
  NOTE: Executing write_package_manifest ...
  DEBUG: Executing python function write_package_manifest
  /OE/build/oe-core/openembedded-core/meta/classes/license_image.bbclass:23: ResourceWarning: unclosed file <_io.TextIOWrapper name='/OE/build/oe-core/tmp-glibc/deploy/licenses/core-image-minimal-qemux86-64/package.manifest' mode='w+' encoding='UTF-8'>
    'w+').write(output)
  ResourceWarning: Enable tracemalloc to get the object allocation traceback
  DEBUG: Python function write_package_manifest finished

[YOCTO #14772]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>