[ffigen] Mention installing xcode command line tools on mac

pkgs/ffigen/README.md

diff --git a/pkgs/ffigen/README.md b/pkgs/ffigen/README.md
index 38c66e5..61464fd 100644
--- a/pkgs/ffigen/README.md
+++ b/pkgs/ffigen/README.md
@@ -76,7 +76,8 @@
 
 #### MacOS
 1. Install Xcode.
-2. Install LLVM - `brew install llvm`.
+2. Install Xcode command line tools - `xcode-select --install`.
+3. Install LLVM - `brew install llvm`.
 
 ## Configurations
 Configurations can be provided in 2 ways-