gl-renderer: Always initialize variable

clang 17 complains that `fourcc` in `gl_renderer_fill_buffer_info()` is
uninitialized in the default case, because it fails to recognize that
if hit, that case will `assert(0)`. To get rid of this complaint, we can
just apply clang's suggestion and initialize the variable with 0 when
declaring it.

Signed-off-by: Max Ihlenfeldt <max@igalia.com>

libweston/renderer-gl/gl-renderer.c

@@ -2393,7 +2393,7 @@ gl_renderer_fill_buffer_info(struct weston_compositor *ec,
 	struct gl_renderer *gr = get_renderer(ec);
 	struct gl_buffer_state *gb = zalloc(sizeof(*gb));
 	EGLint format;
-	uint32_t fourcc;
+	uint32_t fourcc = 0;
 	GLenum target;
 	EGLint y_inverted;
 	bool ret = true;