kernel_headers: Explicitly run headers_install under 'sh'

To avoid build failures in the newer Android build systems,
run the headers_install script explicitly under 'sh'.

Change-Id: If30d2bda653c6be443345a2594e9993d9780db21
Signed-off-by: Soumil Shah <soumshah@codeaurora.org>
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Signed-off-by: Eruvaram Kumar Raja Reddy <erajared@codeaurora.org>