LibC: Add Kernel/API/POSIX/winsize.h to HEADERS

I forgot to do this in be0e1aaf8f.

This makes us actually install that header into the sysroot, unbreaking
ports like openssl, which previously failed to build with:
"fatal error: Kernel/API/POSIX/winsize.h: No such file or directory"

Userland/Libraries/LibC/Headers.cmake

@@ -161,6 +161,7 @@ set(HEADERS
     ../../../Kernel/API/POSIX/ucontext.h
     ../../../Kernel/API/POSIX/termios.h
     ../../../Kernel/API/POSIX/signal.h
+    ../../../Kernel/API/POSIX/winsize.h
 
     ../../../Kernel/API/serenity_limits.h