libsupport: fix function prototype for quota_write_inode()

libsupport: fix function prototype for quota_write_inode()

Commit 2d2d799c7261 ("Clean up codes for adding new quota type")
changed the second paramter of quota_write_inode() from taking a
single quota type to taking a logical OR of (1 << quota_types).

The one thing this commit didn't change was the function prototype for
quota_write_inode() in the header file from an enum to an unsigned
int.  Most C compilers don't seem to mind, and omission is mostly
harmless.  However, mingw64 does issue a warning which gets promoted
to an error.

Fixes: 2d2d799c7261 ("Clean up codes for adding new quota type")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>