scripts: Take only first line of clang version.

Using the line below to compile with Clang:
    ```
    $make ARCH=x86_64 CC=clang -j9
    ```

Emits the following warning:
    ```
    *** libclang (used by the Rust bindings generator 'bindgen') version does not match Clang's. This may be a problem.
    ***   libclang version: 13.0.0
13.0.0                                                                                                                                                        ***   Clang version:    13.0.0
    ***
    ```

This is a result of `$LC_ALL=C bindgen
scripts/rust-is-available-bindgen-libclang.h`, which gets called from
`scripts/rust-is-available.sh`, producing two matches against the
grep'ed pattern "[0-9]+\.[0-9]+\.[0-9]+":
    ```
    scripts/rust-is-available-bindgen-libclang.h:2:9: warning: clang version 13.0.0 (Fedora 13.0.0-3.fc35) [-W#pragma-messages], err: false
    ```
This fixes the issue by pipe'ing the matching result to a `head` call,
getting the first result one.

Signed-off-by: Tiago Lam <tiagolam@gmail.com>

Author: tiagolam

scripts/rust-is-available.sh

@@ -104,6 +104,7 @@ bindgen_libclang_version=$( \
 	LC_ALL=C "$BINDGEN" $(dirname $0)/rust-is-available-bindgen-libclang.h 2>&1 >/dev/null \
 		| grep -F 'clang version ' \
 		| grep -oE '[0-9]+\.[0-9]+\.[0-9]+' \
+		| head -n 1 \
 )
 bindgen_libclang_min_version=$($min_tool_version llvm)
 bindgen_libclang_cversion=$(get_canonical_version $bindgen_libclang_version)