Fix iOS build

R=eroman@chromium.org
BUG=600619

Review URL: https://codereview.chromium.org/1857423002 .
diff --git a/nss.gyp b/nss.gyp
index 2bc7f44..5cc8316 100644
--- a/nss.gyp
+++ b/nss.gyp
@@ -214,6 +214,12 @@
                 'HAVE_SOCKLEN_T',
                 'HAVE_STRERROR',
               ],
+              'sources!': [
+                 # os_Darwin_x86.s and os_Darwin_x86_64.s are included by
+                 # os_Darwin.s.
+                 'nspr/pr/src/md/unix/os_Darwin_x86.s',
+                 'nspr/pr/src/md/unix/os_Darwin_x86_64.s',
+              ],
             }],
             ['component == "static_library"', {
               'defines': [