package/lua-curl: add variant definition of lcurl_url_t

lua-curl has separate code paths for compilers that support forward
typedef declarations and those who don't. For the latter case, one
structure was not properly defined, causing a build failure with older
compilers.

Fixes:

  http://autobuild.buildroot.org/results/8d76ad49837b368a7bba3c3dfd445a035471268a/



Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[Thomas: improve commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>