gnu: pari-gp: Add X11 input.

* gnu/packages/algebra.scm (pari-gp)[inputs]: Add libx11, which enables
  plotting functions.

gnu/packages/algebra.scm

@@ -26,6 +26,7 @@
   #:use-module (gnu packages perl)
   #:use-module (gnu packages readline)
   #:use-module (gnu packages flex)
+  #:use-module (gnu packages xorg)
   #:use-module ((guix licenses) #:prefix license:)
   #:use-module (guix packages)
   #:use-module (guix download)
@@ -125,6 +126,7 @@ solve the shortest vector problem.")
                 "0k1qqagfl6zn7gvwmsqffj6g9yrzqvszwh2mblhmxpjlw1pigfh8"))))
    (build-system gnu-build-system)
    (inputs `(("gmp" ,gmp)
+             ("libx11" ,libx11)
              ("perl" ,perl)
              ("readline" ,readline)))
    (arguments