input_common/sdl/sdl_impl: Mark SDLEventToButtonParamPackage() as static

Its prototype declared at the top of the translation unit contains the
static qualifier, so the function itself should also contain it to make
it a proper internally linked function.

src/input_common/sdl/sdl_impl.cpp

@@ -510,7 +510,7 @@ SDLState::~SDLState() {
     }
 }
 
-Common::ParamPackage SDLEventToButtonParamPackage(SDLState& state, const SDL_Event& event) {
+static Common::ParamPackage SDLEventToButtonParamPackage(SDLState& state, const SDL_Event& event) {
     Common::ParamPackage params({{"engine", "sdl"}});
 
     switch (event.type) {