Fix ODR violation.

src/ports/SkFontMgr_fontconfig.cpp and src/fonts/SkFontMgr_fontconfig.cpp both define SkFontMgr_fontconfig.

BUG=skia:
TBR=bungeman

Review URL: https://codereview.chromium.org/1491943004
diff --git a/BUILD.public b/BUILD.public
index 124714e..d20eb23 100644
--- a/BUILD.public
+++ b/BUILD.public
@@ -97,6 +97,7 @@
         "src/ports/SkFontMgr_custom_directory_factory.cpp",
         "src/ports/SkFontMgr_custom_embedded_factory.cpp",
         "src/ports/SkFontMgr_empty_factory.cpp",
+        "src/ports/SkFontMgr_fontconfig.cpp",
         "src/ports/SkImageDecoder_CG.cpp",
         "src/ports/SkFontMgr_fontconfig_factory.cpp",
         "src/ports/SkImageDecoder_WIC.cpp",