%build-inputs does not exists when cross-compiling,
so use #$(this-package-input ...) instead.

* gnu/packages/tcl.scm
  (tk)[arguments]<#:configure-flags>: Use 'this-package-input'
  instead of '%build-inputs' when cross-compiling.
---
 gnu/packages/tcl.scm | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)