libcap: use more compatible shebang

Patch a script to use a shebang that works on systems that don't have
a /bin/bash, e.g. NixOS or GuixSD.

Signed-off-by: Ilya Katsnelson <me@0upti.me>

package/libs/libcap/patches/900-use-more-compatible-shebang.patch

@@ -0,0 +1,8 @@
+--- a/progs/mkcapshdoc.sh
++++ b/progs/mkcapshdoc.sh
+@@ -1,4 +1,4 @@
+-#!/bin/bash
++#!/usr/bin/env bash
+ # This script generates some C code for inclusion in the capsh binary.
+ # The Makefile generally only generates the .c code and compares it
+ # with the checked in code in the progs directory.