ci: add -Werror gcc flag

.gitlab/build.sh

@@ -7,7 +7,7 @@ if [[ "$SYSTEM" == "linux" ]]; then
   # on linux we perform a coverage analysis
   cmake -B build -S . -DCMAKE_INSTALL_PREFIX=$PWD/install \
         -DBUILD_SHARED_LIBS=ON -DCMAKE_VERBOSE_MAKEFILE=ON \
-        -DCMAKE_C_FLAGS="-O0 -g -fPIC --coverage -Wall -fdiagnostics-show-option -fno-inline" \
+        -DCMAKE_C_FLAGS="-O0 -g -fPIC --coverage -Werror -Wall -fdiagnostics-show-option -fno-inline" \
         -DCMAKE_EXE_LINKER_FLAGS="--coverage" -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
 
 else