Fix error: chosen constructor is explicit in copy-initialization.

The following error occured with clang:
.../glbinding/source/glbinding/source/ContextInfo.cpp:41:16: error: chosen constructor is explicit in copy-initialization return {};

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/set:428:14: note: constructor declared here explicit set(const value_compare& __comp = value_compare())

The small change fixxes the error.