libbtrfsutil: python: use MANIFEST.in for headers

[BUG]
Currently with python3.12, the python binding will always result the
following warning:

    [PY]     libbtrfsutil
/usr/lib/python3.12/site-packages/setuptools/_distutils/extension.py:134: UserWarning: Unknown Extension options: 'headers'
  warnings.warn(msg)

[CAUSE]
In the setup.py which specifies the files to be included into the package,
we use setuptools::Extension to specify the file lists and include paths.

But there is no handling of Extension::headers member, thus resulting the
above warning.

[FIX]
According to the docs of setuptools, MANIFEST.in is the file controlling
what files should be included.
So instead of the non-supported headers, use MANIFEST.in to include the
needed headers.

Signed-off-by: Qu Wenruo <wqu@suse.com>

libbtrfsutil/python/MANIFEST.in

@@ -0,0 +1 @@
+include btrfsutilpy.h

libbtrfsutil/python/setup.py

@@ -97,9 +97,6 @@ module = Extension(
         'qgroup.c',
         'subvolume.c',
     ],
-    headers=[
-        'btrfsutilpy.h'
-    ],
     include_dirs=['..'],
     library_dirs=['../..'],
     libraries=['btrfsutil'],