libsystem_headers: export libaudio_system_headers

These headers are used in headers in libsystem_headers and must be
re-exported.

Test: modules using libsystem_headers don't have to have
libaudio_system_headers include dirs set.
Bug: 33241851

Change-Id: Ic2d842a4c97b3e3fed770249f0cc33a399f3cead

libsystem/Android.bp

@@ -2,6 +2,10 @@ cc_library_headers {
     name: "libsystem_headers",
     vendor_available: true,
     host_supported: true,
+
+    header_libs: ["libaudio_system_headers"],
+    export_header_lib_headers: ["libaudio_system_headers"],
+
     export_include_dirs: ["include"],
 
     target: {