ci: don't fail test kmodule builds on older kernels

We are now getting:

  WARNING: Module.symvers is missing.
           Modules may not have dependencies or modversions.
           You may get many unresolved symbol errors.
           You can set KBUILD_MODPOST_WARN=1 to turn errors into warning
           if you want to proceed at your own risk.

So let's set KBUILD_MODPOST_WARN=1.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

.github/actions/build-selftests/build_selftests.sh

@@ -41,6 +41,7 @@ fi
 cd ${REPO_ROOT}/${REPO_PATH}
 make headers
 make \
+	KBUILD_MODPOST_WARN=1 \
 	CLANG=clang-${LLVM_VERSION} \
 	LLC=llc-${LLVM_VERSION} \
 	LLVM_STRIP=llvm-strip-${LLVM_VERSION} \