use #ifdef instead of #if for SK_VULKAN

Try to fix android roll

Bug: skia:
Change-Id: Iae32027b299d8a975d3caf70f0f8910541ca972d
Reviewed-on: https://skia-review.googlesource.com/19800
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/gpu/GrBackendTextureImageGenerator.cpp b/src/gpu/GrBackendTextureImageGenerator.cpp
index c7e77d6..0b6d64e 100644
--- a/src/gpu/GrBackendTextureImageGenerator.cpp
+++ b/src/gpu/GrBackendTextureImageGenerator.cpp
@@ -33,7 +33,7 @@
                                                          int width, int height,
                                                          GrPixelConfig config,
                                                          GrBackendObject handle) {
-#if SK_VULKAN
+#ifdef SK_VULKAN
     if (kVulkan_GrBackend == backend) {
         GrVkImageInfo* vkInfo = (GrVkImageInfo*)(handle);
         return GrBackendTexture(width, height, *vkInfo);