encoder: allow to build without shaderc dep 2

Fixup of 62138ad

Author: dabrain34

vk_video_encoder/demos/vk-video-enc/CMakeLists.txt

@@ -121,6 +121,10 @@ if(TARGET vulkan)
     list(APPEND definitions PRIVATE -DUNINSTALLED_LOADER="$<TARGET_FILE:vulkan>")
 endif()
 
+if(USE_SHADERC)
+    list(APPEND definitions PRIVATE -DSHADERC_SUPPORT)
+endif()
+
 if(WIN32)
     list(APPEND definitions PRIVATE -DVK_USE_PLATFORM_WIN32_KHR)
     list(APPEND definitions PRIVATE -DWIN32_LEAN_AND_MEAN)