xtask: Add an error message for missing libbf

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>

.gitignore

@@ -4,3 +4,5 @@ target/
 !.vscode/settings.json
 site/
 .idea/
+# Checkout of libbpf, used for codegen + compiling C test code
+libbpf/

test/integration-test/build.rs

@@ -71,6 +71,15 @@ fn main() {
             .unwrap()
             .join("libbpf");
 
+        if !libbpf_dir.exists() {
+            panic!(
+                "Directory does not exist at {:?}\n\
+                Please perform a shallow clone of libbpf:\n\
+                git clone https://github.com/libbpf/libbpf --depth 1\n",
+                libbpf_dir
+            );
+        }
+
         let libbpf_headers_dir = out_dir.join("libbpf_headers");
 
         let mut includedir = OsString::new();