libmnl: fix build when bash is not located at /bin/bash

libmnl: fix build when bash is not located at /bin/bash

This fixes the libmnl build on macOS, which ships with an outdated bash
at /bin/bash. During the OpenWrt build, a modern host bash is built and
made available at staging_dir/host/bin/bash, which is present before
/bin/bash in the build's PATH.

This is similar to 8f7ce3aa6dda, presently appearing at
package/kernel/mac80211/patches/build/001-fix_build.patch.

Signed-off-by: Mark Mentovai <mark@mentovai.com>