Globally enable windows symlinks (#327)

Turns out that `startup:someconfig` is invalid syntax. Since the flag
only applies to Windows it's safe to make it global.

.bazelrc

@@ -51,6 +51,6 @@ build:tsan --cache_test_results=no
 build:tsan --run_under=//tools:tsan.sh
 
 # Configuration options for Windows builds.
-startup:windows --windows_enable_symlinks
+startup --windows_enable_symlinks
 build:windows --cxxopt=/std:c++14 --host_cxxopt=/std:c++14
 build:windows --enable_runfiles