Fix link error when running `make tools` with skia_shared_lib=1

gyp/tools.gyp

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
     # GYP file to build various tools.
 #
 # To build on Linux:
 #  ./gyp_skia tools.gyp && make tools
 #
 {
@@ skipping 171 matching lines @@
         'tools.gyp:picture_utils',
       ],
       'cflags': [
         '-O3',
       ],
       'conditions': [
         [ 'skia_os in ["linux", "freebsd", "openbsd", "solaris", "chromeos"]', {
           'link_settings': {
             'libraries': [
               '-lrt',
+              '-pthread',
             ],
           },
         }],
         ['skia_opencl', {
           'sources': [
             '../tools/skpdiff/SkCLImageDiffer.cpp',
             '../tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp',
           ],
           'conditions': [
             [ 'skia_os == "mac"', {
@@ skipping 569 matching lines @@
               'flags.gyp:flags',
               'skia_lib.gyp:skia_lib',
               'resources',
             ],
           },
         ],
       },
     ],
   ],
 }