shaders/icc: don't spam logs if there is no lcms support

pl_icc_update needs to be called every frame, so if libplacebo is
compiled without lcms support, this leads to spam. There's no need to
warn more than once so just keep track of it in a bool.

src/shaders/icc.c

@@ -758,7 +758,11 @@ bool pl_icc_update(pl_log log, pl_icc_object *obj,
                    const struct pl_icc_profile *profile,
                    const struct pl_icc_params *params)
 {
-    pl_err(log, "libplacebo compiled without LittleCMS 2 support!");
+    static bool warned;
+    if (!warned) {
+        pl_err(log, "libplacebo compiled without LittleCMS 2 support!");
+        warned = true;
+    }
     *obj = NULL;
     return false;
 }