Update the CI to use -Werror

[philberty]

We are using github automations for our PR's. Now that we have working bootstrap build lets avoid regressions here by adding -Werror to our CI

This is how we configure our build for CI automation in PR's

gccrs/.github/workflows/ccpp.yml

Lines 37 to 44 in 28f527c

	- name: Configure
	run: |
	mkdir -p gccrs-build;
	cd gccrs-build;
	../configure \
	--enable-languages=rust \
	--disable-bootstrap \
	--enable-multilib

Let's update this to add --enable-werror to catch warnings breaking the build again.

[tschwinge]

Note that GCC generally is only expected to be "warning-clean" without --disable-bootstrap (that is, default --enable-bootstrap for a native build; see #336), and not for the initial stage where it's using the "initial" compiler -- otherwise we're at the mercy of whatever "initial" compiler we're using.

[CastilloDel]

@tschwinge In the script it says that --enable-werror only enables -Werror in bootstrap stage2 and later (check here). So it would be okay to add that and enable the bootstrap, right?

[CastilloDel]

I made a PR, please let me know if a change is needed! 😄