add xxhash submodule

Building-for-Linux.md

@@ -37,7 +37,7 @@ crew install xorg_nouveau_driver
 1. `git clone https://github.com/dolphin-emu/dolphin`
 2. `cd dolphin`
 
-## Add necessary submodules
+## Add necessary submodules and check for new commits
 ```bash
 git submodule update --init --recursive \
 Externals/mGBA \
@@ -51,6 +51,7 @@ Externals/gtest \
 Externals/rcheevos \
 Externals/fmt \
 Externals/lz4 \
+Externals/xxhash \
 && git pull --recurse-submodules
 ```