Suppress -Wunused-function for nss.

In iOS builds:

../../third_party/nss/nss/lib/freebl/sysrand.c:31:1:
  warning: unused function 'rng_systemFromNoise' [-Wunused-function]
rng_systemFromNoise(unsigned char *dest, size_t maxLen)
^

BUG=573779
R=mark@chromium.org

Review URL: https://codereview.chromium.org/1558903002 .
diff --git a/nss.gyp b/nss.gyp
index 08f9446..206b4bc 100644
--- a/nss.gyp
+++ b/nss.gyp
@@ -1141,6 +1141,9 @@
               '-Wno-switch',
               # nss has some `unsigned < 0` checks.
               '-Wno-tautological-compare',
+              # nss-urandom-abort.patch removed the only call to
+              # rng_systemFromNoise
+              '-Wno-unused-function',
             ],
           },
           'conditions': [