gbm/gbm-symbols-check: do not mandate bash

Analogous to previous commit.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/gbm/gbm-symbols-check

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 FUNCS=$(nm -D --defined-only ${1-.libs/libgbm.so} | grep -o "T .*" | cut -c 3- | while read func; do
 ( grep -q "^$func$" || echo $func )  <<EOF