CMake + MinGW: missing link to ws2_32

[SpaceIm]

In CMakeLists, wslay lib is not linked to ws2_32 if Windows, so building shared lib with MinGW fails with undefined references:

ld.exe: lib/CMakeFiles/wslay_shared.dir/wslay_event.c.obj:wslay_event.c:(.text+0x5cd): undefined reference to `__imp_htons'