[build] add guard to ensure use with linux only

This change adds an assert to verify that the library is built only for
linux. See BLD-572 for details.

Test: CQ
Change-Id: I9a8932c556a8c575d2bf5c6dd85fc6a37cd4fe46
diff --git a/BUILD.gn b/BUILD.gn
index 44bbac1..18e48e9 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -58,6 +58,7 @@
 }
 
 group("libxml2") {
+  assert(is_linux, "${target_name} only builds for linux currently (BLD-572)")
   public_deps = [ ":xml2" ]
 }