Cleanup: add missing #includes to some headers

It should be possible to `#include` any header without having to worry
about its dependencies.

I didn't go and check all include files for this, just the ones that caused
me errors while I was refactoring the `anim_sys.c` file.

No functional changes.

source/blender/blenkernel/BKE_animsys.h

@@ -28,6 +28,8 @@
 extern "C" {
 #endif
 
+#include "BLI_sys_types.h" /* for bool */
+
 struct AnimData;
 struct Depsgraph;
 struct FCurve;

source/blender/blenlib/BLI_compiler_typecheck.h

@@ -24,6 +24,8 @@
  * These depend on compiler extensions and c11 in some cases.
  */
 
+#include "BLI_utildefines_variadic.h"
+
 /* Causes warning:
  * incompatible types when assigning to type 'Foo' from type 'Bar'
  * ... the compiler optimizes away the temp var */