Build a shared lib as well as provide a source set

So that Rust artifacts can link with the shared library.

Change-Id: I69b449d929f7a78a3930416aafb58a7611d2a9de
diff --git a/BUILD.gn b/BUILD.gn
index 8126f94..25ef286 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -36,6 +36,7 @@
   public_deps = [
     ":cairo_install_headers",
     ":cairo_lib",
+    ":cairo_shlib",
   ]
 }
 
@@ -249,3 +250,10 @@
     "//third_party/pixman",
   ]
 }
+
+shared_library("cairo_shlib") {
+  output_name = "libcairo"
+  deps = [
+    ":cairo_lib(//build/toolchain/fuchsia:${target_cpu}-shared)",
+  ]
+}