Quote batch variables to support paths with spaces

With this change, bootstrap and activate complete successfully on
Windows when Pigweed is checked out at a path with spaces.

Change-Id: Ida465f422d706b78e12d10997477226c45d67908
diff --git a/activate.bat b/activate.bat
index 1dbec98..0068dc3 100644
--- a/activate.bat
+++ b/activate.bat
@@ -19,5 +19,5 @@
 :: environment in a shell.
 
 set PW_SKIP_BOOTSTRAP=1
-call %~dp0\bootstrap.bat
+call "%~dp0\bootstrap.bat"
 set PW_SKIP_BOOTSTRAP=