Change cfg(GTK_3_x) to cfg(feature = "GTK_3_x") to make cargo build work

Master branch doesn't let me specify rgtk as dependency in Cargo.toml
because features are not matching.
Master checks with cfg for GTK_3_12 but it should be feature="GTK_3_12".
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Merge pull request #99 from buster/cargo-features-fix

Cargo features fix