gnu: gst-libav: Add missing quotes in phase names passed to 'modify-phases'.

This should have been done in 1ca8ff28.

* gnu/packages/gstreamer.scm (gst-libav): Add missing quotes in phase names
  passed to 'modify-phases'.

gnu/packages/gstreamer.scm

@@ -264,7 +264,7 @@ (define-public gst-libav
     (arguments
      '(#:phases
        (modify-phases %standard-phases
-         (add-before configure patch-/bin/sh
+         (add-before 'configure 'patch-/bin/sh
                      (lambda _
                        (substitute* "gst-libs/ext/libav/configure"
                          (("#! /bin/sh")