build/CI: library `-Wno-error` escape is too coarse-grained

[ppannuto]

libtock-c is fairly pedantic about warnings. When we build external libraries, we relax this by no longer error'ing on warnings (i.e.

libtock-c/libnrfserialization/Makefile

Lines 92 to 93 in 25a49c4

	# Avoid failing in CI due to warnings in the library.
	CPPFLAGS_$(LIBNAME) += -Wno-error

). While this is fine/necessary for the library code itself, it also ignores warnings in our interface code, which in practice lets things like #471 slip through.

We should ideally only allow warnings on actual third-party code, and enforce our stronger checks on our interface code.