samba: patch missing include in libsmbclient.h & update documentation
This patch is backported from Samba 4.13 series - without this include FFmpeg cannot be compiled against libsmbclient.
Updated documentation about dropped "direct" SMB support in mpv and enablement in FFmpeg.
Test Plan: Built ffmpeg against it and checked that mpv smb://remote-file with mpv 0.33 works again.
Reviewers: Triage Team, ermo, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Subscribers: JoshStrobl, ermo
Differential Revision: https://dev.getsol.us/D10022