GDK/Win32: Fix MinGW autotools build

MinGW-w64 CRT provides no 'hid.lib' file. Instead, it has 'libhid.a' which can be linked with '-lhid' linker argument.

Also, we have to declare the '_LIBADD' variable and add 'LDADDS' to it, or 'LDADDS' won't do anything for the build.