Point to merged func_macro feature in bindgen

rust-lang/rust-bindgen#1793 has merged to official master, so point to
it there.

Author: kulp

Cargo.toml

@@ -25,4 +25,4 @@ tar = "0.4"
 libc = "0.2"
 
 [patch.crates-io]
-bindgen = { git = "https://github.com/kulp/rust-bindgen", branch = "function-macros" }
+bindgen = { git = "https://github.com/rust-lang/rust-bindgen", branch = "master" }