Fix ewt_manifest.gyp to support both target and host toolchains

Since base builds for both host and target so much ewt_manifest.
Without this gyp will fail when GYP_CROSSCOMPILE=1 and target_arch=arm:
gyp: Dependency 'S:\src\base\trace_event\etw_manifest\etw_manifest.gyp:etw_manifest#host' not found while trying to load target S:\src\base\base.gyp:base#host

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_nacl_sdk;tryserver.chromium.mac:mac_nacl_sdk
BUG=none

Review URL: https://codereview.chromium.org/1103943002

Cr-Commit-Position: refs/heads/master@{#327379}

base/trace_event/etw_manifest/etw_manifest.gyp

@@ -7,6 +7,7 @@
       # GN version: //base/trace_event/etw_manifest/BUILD.gn
       'target_name': 'etw_manifest',
       'type': 'static_library',
+      'toolsets': ['host', 'target'],
       'conditions': [
         ['OS=="win"', {
           'sources': [