gnu: mesa-headers: Return #t from all phases.

* gnu/packages/gl.scm (mesa-headers)[arguments]: Return #t from the
custom install phase.

gnu/packages/gl.scm

@@ -416,7 +416,8 @@ from software emulation to complete hardware acceleration for modern GPUs.")
            (lambda* (#:key outputs #:allow-other-keys)
              (copy-recursively "include" (string-append
                                           (assoc-ref outputs "out")
-                                                 "/include")))))))))
+                                          "/include"))
+             #t)))))))
 
 ;;; The mesa-demos distribution contains non-free files, many files with no
 ;;; clear license information, and many demos that aren't useful for most