Add ramdisk image sdk trait to cc_library_headers

Allows an sdk to require that a cc_library_headers module provides a
ramdisk image variant for the prebuilt.

Previously, "ramdisk_available: true" would be set in the generated
prebuilt snapshot for any sdk member that specified
"ramdisk_available: true" in the source module. This change will only
add that setting to the snapshot if the ramdisk image variant trait
was explicitly requested for a member.

Bug: 195754365
Test: m nothing
Change-Id: I21e0dd02d380beabf228a72c463fcc603c77d87f
diff --git a/cc/library_headers.go b/cc/library_headers.go
index 4022392..cabeb01 100644
--- a/cc/library_headers.go
+++ b/cc/library_headers.go
@@ -35,6 +35,7 @@
 		HostOsDependent: true,
 		Traits: []android.SdkMemberTrait{
 			nativeBridgeSdkTrait,
+			ramdiskImageRequiredSdkTrait,
 			recoveryImageRequiredSdkTrait,
 		},
 	},