[Emscripten] Add missing gamepad functions

Backends/System/Emscripten/Sources/kinc/backend/gamepad.c.h

@@ -0,0 +1,15 @@
+#include <kinc/input/gamepad.h>
+
+const char *kinc_gamepad_vendor(int gamepad) {
+	return "None";
+}
+
+const char *kinc_gamepad_product_name(int gamepad) {
+	return "Gamepad";
+}
+
+bool kinc_gamepad_connected(int gamepad) {
+	return false;
+}
+
+void kinc_gamepad_rumble(int gamepad, float left, float right) {}

Backends/System/Emscripten/Sources/kinc/backend/html5unit.c

@@ -1,6 +1,7 @@
 #include "audio.c.h"
 #include "display.c.h"
 #include "event.c.h"
+#include "gamepad.c.h"
 #include "mouse.c.h"
 #include "mutex.c.h"
 #include "semaphore.c.h"