[build] Switch off warning on gcc bots.

Switch off maybe-uninitialized warning as it gives often
false positives with optimizations in gcc.

NOTRY=true
TBR=bmeurer@chromium.org, zhengxing.li@intel.com, caitp@igalia.com, ahaas@chromium.org

Review-Url: https://codereview.chromium.org/2425913002
Cr-Commit-Position: refs/heads/master@{#40382}

gypfiles/standalone.gypi

@@ -503,7 +503,10 @@
     },
     'conditions':[
       ['clang==0', {
-        'cflags+': ['-Wno-sign-compare',],
+        'cflags+': [
+          '-Wno-sign-compare',
+          '-Wno-maybe-uninitialized',
+        ],
       }],
       ['clang==1 or host_clang==1', {
         # This is here so that all files get recompiled after a clang roll and