rtl-sdr: fix cross compilation

This patch partially fixes #81985.
Cross-compiling rtl-sdr is possible using this patch.
Use nix build -f <nixpkgs-path> -A pkgsCross.raspberryPi.rtl-sdr to build rtl-sdr.

The error that occured when trying to do this before was as follows:

/nix/store/ldb2v2z0198abkp2minr9a9vz5np4mn4-cmake-3.16.4-armv6l-unknown-linux-gnueabihf/nix-support/setup-hook: line 107: cmake: command not found