Kernel: Move struct winsize definition to API/POSIX/termios.h

POSIX requires it to be defined in this header:
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/termios.h.html
"The <termios.h> header shall define the winsize structure, [...]"

This unbreaks the readline port after 8e34c970b5, which made it
use tc{get,set}winsize() instead of the ioctl.
Previously, it failed to compile with a -Wincompatible-pointer-types
warning:

"expected 'struct winsize *' but argument is of type 'struct winsize *'"

ioctl() takes the argument as a vararg, so it didn't trigger this
warning.

Kernel/API/Ioctl.h

@@ -11,13 +11,6 @@
 extern "C" {
 #endif
 
-struct winsize {
-    unsigned short ws_row;
-    unsigned short ws_col;
-    unsigned short ws_xpixel;
-    unsigned short ws_ypixel;
-};
-
 struct GraphicsConnectorProperties {
     unsigned char multihead_support;
     unsigned char doublebuffer_support;

Kernel/API/POSIX/termios.h

@@ -28,6 +28,14 @@ struct termios {
     speed_t c_ospeed;
 };
 
+// https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/termios.h.html#tag_14_77_03_07
+struct winsize {
+    unsigned short ws_row;
+    unsigned short ws_col;
+    unsigned short ws_xpixel;
+    unsigned short ws_ypixel;
+};
+
 /* c_cc characters */
 #define VINTR 0
 #define VQUIT 1