Export libbenchmark include dir

Export the libbenchmark include directory so the build system
doesn't have manually add it for cc_benchmark modules.

Change-Id: I918a2fa5fb3104f4c2d86930ed2b9c5e00820ec6

benchmarks/Blueprints

@@ -42,6 +42,7 @@ cc_library_static {
     cppflags: benchmark_cppflags,
     srcs: benchmarklib_src_files,
     static_libs: ["libbase"],
+    export_include_dirs: ["."],
     target: {
         darwin: {
             // Only supported on linux systems.