gbm: Fix build without gallium_drm_loader

pipe_loader_drm_probe_fd only exists if HAVE_PIPE_LOADER_DRM is defined.
Patch improved as suggested by Vadim A. Misbakh-Soloviov.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=52962

src/gallium/targets/gbm/gbm.c

@@ -49,11 +49,13 @@ int
 gallium_screen_create(struct gbm_gallium_drm_device *gdrm)
 {
    struct pipe_loader_device *dev;
+#ifdef HAVE_PIPE_LOADER_DRM
    int ret;
 
    ret = pipe_loader_drm_probe_fd(&dev, gdrm->base.base.fd);
    if (!ret)
       return -1;
+#endif /* HAVE_PIPE_LOADER_DRM */
 
    gdrm->screen = pipe_loader_create_screen(dev, get_library_search_path());
    if (gdrm->screen == NULL) {