xwalk.gyp: Unconditionally include xesh's gyp file.

Simply including the gypi file does not have any effect, as one needs to
depend on the target(s) defined there, so we can drop yet another
condition check.

xwalk.gyp

@@ -7,6 +7,7 @@
   'includes' : [
     'xwalk_tests.gypi',
     'application/xwalk_application.gypi',
+    'extensions/xesh/xesh.gypi',
     'xwalk_installer.gypi',
   ],
   'targets': [
@@ -855,9 +856,6 @@
     },
   ], # targets
   'conditions': [
-    ['OS=="linux"', {
-      'includes': [ 'extensions/xesh/xesh.gypi' ],
-    }],
     ['OS=="mac"', {
       'targets': [
         {