Disable warnings about unused functions.

Clang triggers -Wunused-function for get_dir_itr_imp_extra_data, which
may or may not be used depending on the target platform. Better to disable
the warning rather than add macro checks.

include/boost/filesystem/detail/header.hpp

@@ -37,6 +37,8 @@
 #pragma GCC diagnostic push
 // unused parameter 'arg'
 #pragma GCC diagnostic ignored "-Wunused-parameter"
+// unused function 'foo'
+#pragma GCC diagnostic ignored "-Wunused-function"
 
 #endif