Wayland: Make text input logic static

Since the seat listener was moved into wl_window.c there is no reason
for text input to have external linkage.

(cherry picked from commit a1a73ee862)

src/wl_platform.h

@@ -366,7 +366,6 @@ typedef struct _GLFWcursorWayland
 
 void _glfwAddOutputWayland(uint32_t name, uint32_t version);
 void _glfwUpdateContentScaleWayland(_GLFWwindow* window);
-GLFWbool _glfwInputTextWayland(_GLFWwindow* window, uint32_t scancode);
 
 void _glfwAddSeatListenerWayland(struct wl_seat* seat);
 void _glfwAddDataDeviceListenerWayland(struct wl_data_device* device);