cmake: move missing-declarations warning to top-level

We should enforce declarations throughout the code-base, including
examples, fuzzers and tests, not just in the `src` tree.

cmake/DefaultCFlags.cmake

@@ -125,6 +125,7 @@ else()
 	enable_warnings(documentation)
 	disable_warnings(documentation-deprecated-sync)
 	disable_warnings(missing-field-initializers)
+	enable_warnings(missing-declarations)
 	enable_warnings(strict-aliasing)
 	enable_warnings(strict-prototypes)
 	enable_warnings(declaration-after-statement)

src/CMakeLists.txt

@@ -41,8 +41,6 @@ set(LIBGIT2_INCLUDES
 set(LIBGIT2_SYSTEM_INCLUDES "")
 set(LIBGIT2_LIBS "")
 
-enable_warnings(missing-declarations)
-
 if(HAVE_FUTIMENS)
 	set(GIT_USE_FUTIMENS 1)
 endif ()