gnu: btrfs-progs: Update to 4.7.

* gnu/packages/linux.scm (btrfs-progs): Update to 4.7.
[native-inputs]: Add missing header comment for which.

gnu/packages/linux.scm

@@ -2598,7 +2598,7 @@ and copy/paste text in the console and in xterm.")
 (define-public btrfs-progs
   (package
     (name "btrfs-progs")
-    (version "4.6.1")
+    (version "4.7")
     (source (origin
               (method url-fetch)
               (uri (string-append "mirror://kernel.org/linux/kernel/"
@@ -2606,7 +2606,7 @@ and copy/paste text in the console and in xterm.")
                                   "btrfs-progs-v" version ".tar.xz"))
               (sha256
                (base32
-                "06c9l6m3w29dndk17jrlpgr01wykl10h34zva8zc2c571z6mrlaf"))))
+                "1kwr54mxlw8yir14w27p8xqxfr51ir6gd5nmn0wiwri13jqdlpqs"))))
     (build-system gnu-build-system)
     (outputs '("out"
                "static"))      ; static versions of binaries in "out" (~16MiB!)
@@ -2637,6 +2637,7 @@ and copy/paste text in the console and in xterm.")
                      ("libxml2" ,libxml2)
                      ("docbook-xml" ,docbook-xml)
                      ("docbook-xsl" ,docbook-xsl)
+                     ;; For tests
                      ("which" ,which)))
     (home-page "https://btrfs.wiki.kernel.org/")
     (synopsis "Create and manage btrfs copy-on-write file systems")