Allow SkiaVkRenderEngine to not be compiled

A device will use RenderEngine with either SkiaVk or SkiaGL, but never
both at the same time.  Given that the choice is controlled by a
build-time value in the device's "device.mk" file, to save space, we
should allow the RenderEngine back-ends to be optionally built.  For
now, we'll only do this for SkiaVkRenderEngine.

Test: Compile
Bug: 259323498
Change-Id: Id547feaee7e8aeb4e46759d3d7c9c5dbe18f5211
diff --git a/libs/renderengine/skia/SkiaVkRenderEngine.cpp b/libs/renderengine/skia/SkiaVkRenderEngine.cpp
index f9424f0..2b8495c 100644
--- a/libs/renderengine/skia/SkiaVkRenderEngine.cpp
+++ b/libs/renderengine/skia/SkiaVkRenderEngine.cpp
@@ -14,6 +14,10 @@
  * limitations under the License.
  */
 
+// Allow the SkiaVkRenderEngine class to not be compiled, to save space
+// NOTE: In order to build this class, define `RE_SKIAVK` in a build file.
+#ifdef RE_SKIAVK
+
 // #define LOG_NDEBUG 0
 #undef LOG_TAG
 #define LOG_TAG "RenderEngine"
@@ -673,3 +677,4 @@
 } // namespace skia
 } // namespace renderengine
 } // namespace android
+#endif // RE_SKIAVK