HTHstMan.h is missing #include <netinet/in.h> or #include "wwwsys.h" or both

I was porting this old mosaic to new libwww (5.4) and i encountered this
.../libwww2/HTHstMan.h:94: error: field 'sock_addr' has incomplete type among
other errors, It was fixed by adding those lines. Surely it can be my fault
as I am still learning C or more learning to program C on unix platforms but
for my knowledge
there should be both lines as there is reference to wwwsys.h but no include
to it.