xtask: move libbpf submodule

.github/workflows/gen.yml

@@ -12,7 +12,7 @@ jobs:
           submodules: recursive
 
       - name: update libbpf
-        working-directory: libbpf
+        working-directory: xtask/libbpf
         run: |
           set -e
           git fetch origin

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "libbpf"]
-	path = libbpf
+	path = xtask/libbpf
 	url = https://github.com/libbpf/libbpf

xtask/src/lib.rs

@@ -14,4 +14,4 @@ pub fn exec(cmd: &mut Command) -> Result<()> {
     }
 }
 
-pub const LIBBPF_DIR: &str = "libbpf";
+pub const LIBBPF_DIR: &str = "xtask/libbpf";