[gn build] Fix defines define on Windows

[smeenai]

Summary

On Windows, we won't go into the host_os != "win" block, so defines
won't have been defined, and we'll run into an undefined identifier
error when we try to later append to it. Unconditionally define it at
the start and append to it everywhere else.

[thakis]

Thanks!

llvm/utils/gn/build/BUILD.gn

 import("//llvm/utils/gn/build/buildflags.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 config("compiler_defaults") {
+  defines = []
+
   # FIXME: Don't define this globally here.
   if (host_os != "win") {
-    defines = [ "LLVM_ON_UNIX" ]
+    defines += [ "LLVM_ON_UNIX" ]
   }
 
   if (!llvm_enable_assertions) {
     defines += [ "NDEBUG" ]
   }
 
   cflags = []