gnu: perf: Set 'CC'.

* gnu/packages/linux.scm (perf)[arguments]: Add "CC=gcc" to #:make-flags.
This fixes a (harmless?) error message during the 'configure' phase.

gnu/packages/linux.scm

@@ -3711,6 +3711,7 @@ in a digital read-out.")
              #t)))
        #:make-flags (list (string-append "prefix="
                                          (assoc-ref %outputs "out"))
+                          "CC=gcc"
                           "WERROR=0"
 
                           ;; By default, 'config/Makefile' uses lib64 on