test/common: add shebang to autotest_common.sh.

Without the shebang, the builtin file command assumes autotest_common.sh
is a c file. This is obviously not the case, but it messes up file type
checking when we are confirming that all files are checked against scanbuild.

Signed-off-by: Seth Howell <seth.howell@intel.com>
Change-Id: I45f32b23ef01b07ce9b72caed743e6670b99055e
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1311


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>