gn: Disable warning as error but not disable completely

This will still emit the diagnostic but it wont break the build,
additionally pass it to CXXFLAGS since thats the right subset to
disable it for.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-devtools/gn/gn_git.bb

@@ -16,7 +16,7 @@ B = "${WORKDIR}/build"
 # ../git/src/gn/desc_builder.cc: In member function 'base::Value {anonymous}::BaseDescBuilder::ToBaseValue(const Scope*)':
 # ../git/src/gn/desc_builder.cc:179:21: error: redundant move in return statement [-Werror=redundant-move]
 #   179 |     return std::move(res);
-CFLAGS:append = " -Wno-redundant-move"
+CXXFLAGS += "-Wno-error=redundant-move"
 
 # Map from our _OS strings to the GN's platform values.
 def gn_platform(variable, d):