perf: unbreak build since glibc 2.24 upgrade

glibc 2.24 deprecated readdir_r, breaking the perf build:

  $ nix-build -A linuxPackages.perf
  ...
    CC       util/event.o
    CC       util/evlist.o
  util/event.c: In function '__event__synthesize_thread':
  util/event.c:448:2: error: 'readdir_r' is deprecated [-Werror=deprecated-declarations]
    while (!readdir_r(tasks, &dirent, &next) && next) {
    ^
  In file included from /nix/store/8ic0jwg3p5vcwx52k4781n987hmv0bks-glibc-2.24-dev/include/features.h:368:0,
                   from /nix/store/8ic0jwg3p5vcwx52k4781n987hmv0bks-glibc-2.24-dev/include/stdint.h:25,
                   from /nix/store/jsazxc1b86g2ww569ziwhhvkz8z43vjd-gcc-5.4.0/lib/gcc/x86_64-unknown-linux-gnu/5.4.0/include/stdint.h:9,
                   from /tmp/nix-build-perf-linux-4.4.19.drv-0/linux-4.4.19/tools/include/linux/types.h:6,
                   from util/event.c:1:
  /nix/store/8ic0jwg3p5vcwx52k4781n987hmv0bks-glibc-2.24-dev/include/dirent.h:189:12: note: declared here
   extern int __REDIRECT (readdir_r,
              ^
  util/event.c: In function 'perf_event__synthesize_threads':
  util/event.c:586:2: error: 'readdir_r' is deprecated [-Werror=deprecated-declarations]
    while (!readdir_r(proc, &dirent, &next) && next) {

Fix by adding -Wno-error=deprecated-declarations compile flag.