Update WATCHLISTS for ninja, siso and reclient changes

This is for
https://crrev.com/c/4633180/3#message-efdd6b642c3eadb8ee746085f2750f6077487f5d

Change-Id: I4a108d9e9ec439113c215b0a7af8c815cb67da09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4653532
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>

WATCHLISTS

@@ -14,6 +14,9 @@
     'depot_tools': {
       'filepath': '.+',
     },
+    'build': {
+      'filepath': '.*(ninja|reclient|siso).*',
+    },
   },
 
   'WATCHLISTS': {
@@ -21,6 +24,9 @@
     'depot_tools': [
       'chops-source-team-auto+depot_tools@google.com',
     ],
+    'build': [
+      'chrome-build-team@google.com',
+    ],
   },
 
 }