Fixed build on GCC < 5.1 and Clang.

Apparently I can't do a change without breaking stuff somewhere. Sigh.

src/Magnum/Context.cpp

@@ -547,7 +547,7 @@ Context::Context(Context&& other): _version{std::move(other._version)},
     #ifndef MAGNUM_TARGET_WEBGL
     _flags{std::move(other._flags)},
     #endif
-    _extensionRequiredVersion{std::move(other._extensionRequiredVersion)},
+    _extensionRequiredVersion(std::move(other._extensionRequiredVersion)),
     _extensionStatus{std::move(other._extensionStatus)},
     _supportedExtensions{std::move(other._supportedExtensions)},
     _state{std::move(other._state)},