targets: Add stub implementations of xrt_gfx_provider_create_fd

This is needed by st/prober's default implementation of xrt_instance.

src/xrt/targets/cli/cli_main.c

@@ -51,3 +51,10 @@ main(int argc, const char **argv)
 	}
 	return cli_print_help(argc, argv);
 }
+
+//! Needed to support st/prober.
+int
+xrt_gfx_provider_create_fd()
+{
+	return -1;
+}

src/xrt/targets/gui/gui_sdl2_main.c

@@ -57,3 +57,10 @@ main(int argc, char **argv)
 
 	return 0;
 }
+
+//! Needed to support st/prober.
+int
+xrt_gfx_provider_create_fd()
+{
+	return -1;
+}