Add vendor_available: true to libbuildverison

This allows using libbuildversion in Cuttlefish, which builds many
binaries for both vendor and host.

On vendor executables, this gets the build id from the
ro.vendor.build.version.incremental property.

Bug: 206843934
Test: m -j
Change-Id: I9ab11b42de3f1febf282f107c3e137592097d3c0

cc/libbuildversion/Android.bp

@@ -19,4 +19,5 @@ cc_library_static {
         "//apex_available:platform",
         "//apex_available:anyapex",
     ],
+    vendor_available: true,
 }

cc/libbuildversion/libbuildversion.cpp

@@ -36,7 +36,11 @@ std::string GetBuildNumber() {
     return soong_build_number;
   }
 
+#ifdef __ANDROID_VENDOR__
+  const prop_info* pi = __system_property_find("ro.vendor.build.version.incremental");
+#else
   const prop_info* pi = __system_property_find("ro.build.version.incremental");
+#endif
   if (pi == nullptr) return "";
 
   std::string property_value;