CMakeLists: Enable -Wmissing-declarations on Linux builds

Allows catching cases where internal linkage isn't specified for helper
functions when they should be marked as such.

src/CMakeLists.txt

@@ -56,6 +56,7 @@ else()
         -Werror=implicit-fallthrough
         -Werror=reorder
         -Wextra
+        -Wmissing-declarations
         -Wno-attributes
         -Wno-unused-parameter
     )