gnu: lxc: Adjust for GCC 7.

* gnu/packages/virtualization.scm (lxc)[arguments]: Add #:make-flags.

gnu/packages/virtualization.scm

@@ -381,6 +381,12 @@ all common programming languages.  Vala bindings are also provided.")
                             "/share/doc/" ,name "-" ,version)
              "--sysconfdir=/etc"
              "--localstatedir=/var")
+       #:make-flags
+       ;; Treat the kernel headers as system headers to silence
+       ;; compiler warnings from those.
+       (list (string-append "C_INCLUDE_PATH="
+                            (assoc-ref %build-inputs "kernel-headers")
+                            "/include"))
        #:phases
        (modify-phases %standard-phases
          (replace 'install