Use current_os instead of current_toolchain.

This allows us to be less strict regarding which linux toolchain is
used.

Test: fx build
Change-Id: I888a5eeb5b41db1c2e211f757ba556367dbd91e2
diff --git a/include/BUILD.gn b/include/BUILD.gn
index fed4a24..1082a78 100644
--- a/include/BUILD.gn
+++ b/include/BUILD.gn
@@ -26,7 +26,7 @@
 }
 
 config("vulkan_public_config") {
-  if ("$current_toolchain" != "//build/toolchain:linux_x64") {
+  if (current_os == "fuchsia") {
     defines = [ "VK_USE_PLATFORM_FUCHSIA=1" ]
   }
 }