functional.h: acquire GIL before calling Python function

include/pybind11/functional.h

@@ -24,6 +24,7 @@ public:
             return false;
         object src(src_, true);
         value = [src](Args... args) -> Return {
+            gil_scoped_acquire acq;
             object retval(src.call(std::move(args)...));
             /* Visual studio 2015 parser issue: need parentheses around this expression */
             return (retval.template cast<Return>());