Add missing shbang line to shell script

If it's executable, it should specify how it's to be executed.

Change-Id: If5671712da3e1fbc42b15d22c1253129910091bc
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

tests/manual/rhi/displacement/buildshaders

@@ -1,3 +1,4 @@
+#!/bin/sh
 qsb --glsl 320es,410 --hlsl 50 --msl 12 --msltess material.vert -o material.vert.qsb
 qsb --glsl 320es,410 --hlsl 50 --msl 12 material.frag -o material.frag.qsb
 qsb --glsl 320es,410 --msl 12 --tess-mode triangles material.tesc -o material.tesc.qsb