Build with -Wmissing-declarations (#71)

This flag is used to find global functions defined without a previous
declaration. Usually it spots accidental globals, but in this case it
was just missing headers.

CMakeLists.txt

@@ -142,6 +142,7 @@ foreach(lib screen dom component)
     target_compile_options(${lib} PRIVATE "-Wextra")
     target_compile_options(${lib} PRIVATE "-pedantic")
     target_compile_options(${lib} PRIVATE "-Werror")
+    target_compile_options(${lib} PRIVATE "-Wmissing-declarations")
     target_compile_options(${lib} PRIVATE "-Wno-sign-compare")
   endif()