%build-inputs does not exist when cross-compiling, so
use this-package-input instead.

* gnu/packages/readline.scm
  (readline)[arguments]<#:configure-flags>: Use this-package-input
  instead of %build-inputs.
---
 gnu/packages/readline.scm | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)