GN: get echo-headers sources via exec_script

gyp/fuzz.gyp

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
     'includes': [ 'apptype_console.gypi' ],
     'targets': [{
         'target_name': 'fuzz',
         'type': 'executable',
         'defines': [
           'SK_FUZZ_LOGGING',
         ],
         'sources': [
-          '<!@(python find.py ../fuzz "*.cpp")',
+          '<!@(python find.py "*.cpp" ../fuzz)',
           '../tests/PathOpsDebug.cpp',
         ],
         'dependencies': [
             'flags.gyp:flags',
             'skia_lib.gyp:skia_lib',
         ],
         'include_dirs': [
             '../include/private',
             '../src/core',
             '../src/pathops',
         ],
     }],
 }