libutils: export libbacktrace headers

This is used in CallStack.h.

Test: m -j libutils
Test: links
Test: trying to compile with BOARD_VNDK_VERSION := current
Bug: 33241851
Change-Id: I076c9551d356824d6e69b82349b890369bc7eea3
Merged-In: If7258e7b38000a72fd520f725030fb0038f90167

libbacktrace/Android.bp

@@ -57,6 +57,7 @@ libbacktrace_sources = [
 
 cc_library_headers {
     name: "libbacktrace_headers",
+    vendor_available: true,
     export_include_dirs: ["include"],
 }
 

libutils/Android.bp

@@ -20,7 +20,12 @@ cc_library_headers {
     header_libs: ["libsystem_headers",],
     export_header_lib_headers: ["libsystem_headers",],
     export_include_dirs: ["include"],
+
     target: {
+        android: {
+            header_libs: ["libbacktrace_headers"],
+            export_header_lib_headers: ["libbacktrace_headers"],
+        },
         linux_bionic: {
             enabled: true,
         },