Third-party: Disable -Werror for flatcc build #5146

Job	Run time
test-arm / test-backend-linux (arm_tosa_fp, operators) / linux-job	23m 28s
test-arm / test-backend-linux (arm_vgf_fp, operators) / linux-job	22m 39s
test-arm / test-backend-linux (arm_vgf_fp, models) / linux-job	19m 49s
test-arm / test-backend-linux (arm_tosa_fp, models) / linux-job	18m 4s
test-arm / test-backend-macos	-1s
test-arm / package-golden-artifacts	43s
	1h 24m 42s

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