[style] rename header file to locator_getters.hpp (#6422)

This commit renames the `Locator` header file to follow the same style
used by other core modules (i.e., use underscore between words in a
file name).

src/core/api/thread_api.cpp

@@ -39,7 +39,7 @@
 
 #include "common/debug.hpp"
 #include "common/instance.hpp"
-#include "common/locator-getters.hpp"
+#include "common/locator_getters.hpp"
 #include "common/settings.hpp"
 
 using namespace ot;