Roll buildroot to 6669621484b649b9355fb2dd37950daf3d2ac65b (#18652)

flutter/buildroot@6669621484b649b9355fb2dd37950daf3d2ac65b

See flutter/buildroot#372
diff --git a/DEPS b/DEPS
index 4487c35..05cadd2 100644
--- a/DEPS
+++ b/DEPS
@@ -138,7 +138,7 @@
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'a4f3c4d5023e080ee50596e6623d179e9c5f839b',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '6669621484b649b9355fb2dd37950daf3d2ac65b',
 
    # Fuchsia compatibility
    #