gnu: perf: Really build without '-Werror'.

Until this change, it would fail to build with:

  In file included from /tmp/guix-build-perf-5.2.15.drv-0/linux-5.2.15/tools/include/uapi/linux/ethtool.h:19:0,
		   from xsk.c:18:
  /gnu/store/…-linux-libre-headers-4.19.56/include/linux/if_ether.h:165:1: error: packed attribute is unnecessary for 'ethhdr' [-Werror=packed]

* gnu/packages/linux.scm (perf)[arguments]: In 'configure' phase, remove
"-Werror" from 'tools/lib/bpf/Makefile'.

gnu/packages/linux.scm

@@ -3082,6 +3082,10 @@ in a digital read-out.")
        (modify-phases %standard-phases
          (replace 'configure
            (lambda* (#:key inputs #:allow-other-keys)
+             ;; Don't build with '-Werror', really.
+             (substitute* "tools/lib/bpf/Makefile"
+               (("-Werror") ""))
+
              (setenv "SHELL_PATH" (which "bash"))
              (chdir "tools/perf")
              #t)))