static-code-analysis/cppcheck: suppress unmatched suppressions

The cppcheck invocation without suppression of unmatched suppressions
expects that there would be at least one error of suppressed kind. If there
are none, cppcheck fails as well.
This patch enables ignoring errors of that kind.

Signed-off-by: Maxim Menshikov <maxim.menshikov@iopsys.eu>

static-code-analysis.yml

 variables:
   RUN_FLAWFINDER: "flawfinder --falsepositive -m 4"
-  RUN_CPPCHECK: "cppcheck --enable=all --error-exitcode=1 --suppress=unusedFunction --suppress=unreadVariable ."
+  RUN_CPPCHECK: "cppcheck --enable=all --error-exitcode=1 --suppress=unusedFunction --suppress=unreadVariable --suppress=unmatchedSuppression ."
   RUN_CPD: "/home/user/pmd-bin-6.19.0/bin/run.sh cpd --minimum-tokens 200 --language c --exclude ./test/cmocka --files"
   SOURCE_FOLDER: "src"
   COMMON_IMAGE: "iopsys/code-analysis:0.27"