libutils: export system_headers

vendor + vendor_available modules built with BOARD_VNDK_VERSION :=
current aren't allowed to use global headers. Since libutils uses
system headers itself, it should export them as well.

Bug: 33241851
Test: building with BOARD_VNDK_VERSION := current produces many fewer
errors.

Change-Id: Icc7d2beb2a17da291434c6282eb1b9eb458f114c
Merged-In: I950821f1dae3ddc110db50869247913d5b1aa638

libsystem/Android.bp

@@ -1,4 +1,15 @@
 cc_library_headers {
     name: "libsystem_headers",
+    vendor_available: true,
+    host_supported: true,
     export_include_dirs: ["include"],
+
+    target: {
+        linux_bionic: {
+            enabled: true,
+        },
+        windows: {
+            enabled: true,
+        },
+    }
 }

libutils/Android.bp

@@ -16,6 +16,9 @@ cc_library_headers {
     name: "libutils_headers",
     vendor_available: true,
     host_supported: true,
+
+    header_libs: ["libsystem_headers",],
+    export_header_lib_headers: ["libsystem_headers",],
     export_include_dirs: ["include"],
     target: {
         linux_bionic: {