Fixes GetClipboardText() memory leak for PLATFORM_DESKTOP_SDL

[asdqwe]

SDL_GetClipboardText() must be freed with SDL_free() (doc), but that can't/shouldn't be done to a returned const.

Fix can be tested with:

#include "raylib.h"
int main(void) {
    InitWindow(800, 450, "test");
    SetTargetFPS(60);
    while (!WindowShouldClose()) {

        if (IsKeyPressed(KEY_ONE)) { TraceLog(LOG_INFO, "%s", GetClipboardText()); }

        BeginDrawing();
        ClearBackground(RAYWHITE);
        EndDrawing();
    }
    CloseWindow();
    return 0;
}

[raysan5]

@asdqwe Great improvement! Thanks!