gnu: cross-base: Add srfi-26 for glibc.

Fixes a regression introduced in
commit efc4eb1475.

* gnu/packages/cross-base.scm (cross-libc)[arguments]: Add (srfi
srfi-26).

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/cross-base.scm

@@ -323,6 +323,11 @@ XBINUTILS and the cross tool chain."
            ;; itself.
            #:implicit-cross-inputs? #f
 
+           ;; We need SRFI 26.
+           #:modules ((guix build gnu-build-system)
+                      (guix build utils)
+                      (srfi srfi-26))
+
            ,@(package-arguments glibc))
        ((#:configure-flags flags)
         `(cons ,(string-append "--host=" target)