CMakeLists: Make missing declarations a compile-time error

Ensures that our code always has its linkage explicit.

src/CMakeLists.txt

@@ -54,6 +54,7 @@ else()
     add_compile_options(
         -Wall
         -Werror=implicit-fallthrough
+        -Werror=missing-declarations
         -Werror=reorder
         -Wextra
         -Wmissing-declarations