.gitignore: Prefix ignored root directories with '/'

If this prefix is omitted, then any subdirectory that matches the pattern will also be ignored.

Change-Id: I09b2629156416c26f885d765f88e69be236b7d3e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/64745
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

.gitignore

@@ -1,28 +1,28 @@
 *.pyc
 
 # Directories added by gclient sync and the GN build
-.cipd
+/.cipd
-.gclient
+/.gclient
-.gclient_entries
+/.gclient_entries
-build
+/build
-buildtools
+/buildtools
-testing
+/testing
-third_party/angle/
+/third_party/angle
-third_party/clang-format/
+/third_party/clang-format
-third_party/glfw/
+/third_party/glfw
-third_party/glm/
+/third_party/glm
-third_party/googletest/
+/third_party/googletest
-third_party/jinja2/
+/third_party/jinja2
-third_party/jsoncpp/
+/third_party/jsoncpp
-third_party/llvm-build
+/third_party/llvm-build
-third_party/markupsafe/
+/third_party/markupsafe
-third_party/swiftshader/
+/third_party/swiftshader
-third_party/tint/
+/third_party/tint
-third_party/vulkan-deps/
+/third_party/vulkan-deps
-third_party/vulkan_memory_allocator/
+/third_party/vulkan_memory_allocator
-third_party/zlib/
+/third_party/zlib
-tools
+/tools
-out
+/out
 
 # Modified from https://www.gitignore.io/api/vim,macos,linux,emacs,windows,sublimetext,visualstudio,visualstudiocode