Skip default dependencies for header libraries

Header libraries don't compile anything, so they don't need the
default stl and libclang_rt dependencies.  Trimming the dependencies
simplifies using header libraries for LLNDK libraries, which need
extra variants that don't exist for the default dependencies.

Test: m checkbuild
Change-Id: Ib9864c3cc7048b2cc52acf304830777b65a51877
diff --git a/cc/stl.go b/cc/stl.go
index 4f8865f..75921c6 100644
--- a/cc/stl.go
+++ b/cc/stl.go
@@ -62,6 +62,8 @@
 		s := ""
 		if stl.Properties.Stl != nil {
 			s = *stl.Properties.Stl
+		} else if ctx.header() {
+			s = "none"
 		}
 		if ctx.useSdk() && ctx.Device() {
 			switch s {