Where is sys/random.h or getrandom()?

I'm trying to build some software that uses the C function getrandom() from the header sys/random.h. (I'm building an image for OpenWRT 18.06 from source.) However compilation always fails at the point where sys/random.h is included because it can't find the header (and indeed, it's not in staging_dir/target-mipsel_24kc_musl/usr/include/).

As far as I can tell this should be available from Linux 3.13 onwards, and 18.06 has 4.14. I see other software in the package repo seems(?) to use sys/random.h eg. Dropbear (I am not sure about this TBH). The Busybox util getrandom is available; as is the device node /dev/urandom which is what getrandom() uses under the hood.

If the getrandom() system call is not available I can rewrite the calls, but I'd really like to know if this is actually just due to a missing dependency or configuration option.