Disable -ffunction-sections for pixman

Release builds in Fuchsia define -ffunction-sections, which causes
the link of the combined cairo/pixman library to fail on x86_64.

Change-Id: Ib6e1fc7afd63f9c21e2b6ce30cdb9bf66082b613
diff --git a/BUILD.gn b/BUILD.gn
index a53ff4f..14f0c18 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -56,6 +56,9 @@
     "-Wno-unused-const-variable",
     "-Wno-unused-local-typedef",
     "-Wno-unused-function",
+    # Release builds in Fuchsia define -ffunction-sections, which causes
+    # the link of the combined cairo/pixman library to fail on x86_64.
+    "-fno-function-sections",
   ]
 
   if (current_cpu == "arm64") {