Support aidl cpp generation

Ideally we'd calculate the headers that are written here too, but I'll
add that in a later change that actually enforces the generated header
list.

Test: mmma -j system/tools/aidl
Change-Id: Ifd2e8e8ff444b0f67270fb5156e7bf7bceddb6be
diff --git a/cc/builder.go b/cc/builder.go
index ee53f41..d0b21ed 100644
--- a/cc/builder.go
+++ b/cc/builder.go
@@ -194,6 +194,7 @@
 	protoFlags  string
 	tidyFlags   string
 	yasmFlags   string
+	aidlFlags   string
 	toolchain   config.Toolchain
 	clang       bool
 	tidy        bool