gnu: qemu: Add libcacard as an input to enable smartcard support.

This fixes an error in gnome-boxes when starting virtual machines:

  Unable to start domain: unsupported configuration: this QEMU binary lacks
  smartcard passthrough mode support

* gnu/packages/virtualization.scm (qemu)[inputs]: Add libcacard.

gnu/packages/virtualization.scm

@@ -223,6 +223,7 @@ exec smbd $@")))
        ("gtk+" ,gtk+)
        ("libaio" ,libaio)
        ("libattr" ,attr)
+       ("libcacard" ,libcacard)     ; smartcard support
        ("libcap" ,libcap)           ; virtfs support requires libcap & libattr
        ("libdrm" ,libdrm)
        ("libepoxy" ,libepoxy)