gnu: perf: Treat kernel headers as system headers.

* gnu/packages/linux.scm (perf)[arguments]: Instead of disabling -Werror,
set C_INCLUDE_PATH.

gnu/packages/linux.scm

@@ -3091,15 +3091,16 @@ 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)))
        #:make-flags (list (string-append "prefix="
                                          (assoc-ref %outputs "out"))
+                          ;; Make sure the kernel headers are treated as system
+                          ;; headers to suppress warnings from those.
+                          (string-append "C_INCLUDE_PATH="
+                                         (assoc-ref %build-inputs "kernel-headers")
+                                         "/include")
                           "WERROR=0"
 
                           ;; By default, 'config/Makefile' uses lib64 on