[GTK] Add WTFURL source files to the build
https://bugs.webkit.org/show_bug.cgi?id=108215

Reviewed by Benjamin Poulain.

.: 

* Source/autotools/symbols.filter: Force the export of the KURL::string() symbol.
This is required when using the WTFURL backend but otherwise doesn't affect the build.

Source/WebCore: 

* platform/KURLWTFURL.cpp:
(WebCore): Only use the stub implementation of the fileSystemPath method for the
Apple ports, other ports should for now still rely on their platform-specific implementations.

Source/WTF: 

The WTFURL implementation sources are all still guarded by the USE(WTFURL) guard,
meaning that the GTK port still uses the default KURL backend. To use the WTFURL
backend instead, one would have to define WTF_USE_WTFURL in Platform.h.

* GNUmakefile.am: List the directories from which source headers will be included.
Plenty of WTFURL code currently just includes the required header by name rather
than specifying the header path as relative to Source/WTF. In the future all the inclusions
should probaby be changed to include the header through the path relative to Source/WTF.
* GNUmakefile.list.am: Add build targets for the WTFURL source files.
* wtf/url/api/ParsedURL.cpp: Specify the complete path to the required headers and
reorder the inclusions.
* wtf/url/src/URLCanon.h: Ditto.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141622 268f45cc-cd09-0410-ab3c-d52691b4dbfc