Add DCCP constant definitions.

libc-test/build.rs

src/unix/bsd/netbsdlike/netbsd/mod.rs

src/unix/notbsd/android/mod.rs

src/unix/notbsd/linux/mips/mod.rs

src/unix/notbsd/linux/musl/mod.rs

src/unix/notbsd/linux/other/mod.rs