jabber_http_file_upload: fix build with gcc 10

[a-wai]

With GCC 10, HFUJabberStreamDataTable being declared in a header file
leads to the creation of 2 objects with that name: one when compiling
hfu_disco.c, the other one when compiling jabber_http_file_upload.c.

This results in link-time errors, which can be fixed by declaring
HFUJabberStreamDataTable as extern.

[Junker]

why do you redefine HFUJabberStreamDataTable in src/jabber_http_file_upload.c ?
Seems it's not necessary with extern.

[a-wai]

Not defining HFUJabberStreamDataTable in the .c file would indeed work, at least with gcc >= 9, but I wouldn't consider it a good practice: extern means it's a declaration (pretty much like a function declaration in a header file), but proper C requires a corresponding definition somewhere, usually in a source file (again, very much like you would do with a function).

gcc seems to handle the lack of proper definition without complaining, but in order to avoid potential issues with other compilers, I think it's best to have HFUJabberStreamDataTable defined in src/jabber_http_file_upload.c

[Junker]

thanks!