Fix passing debug options for gcc

Fix a bug in the test where we were missing "additional_flags=",
causing -gstatement-frontiers not to be passed to the compiler.

The issue was introduced in
eb24648c ("Fix gdb.cp/step-and-next-inline.exp with Clang").

gdb/testsuite:
2020-12-31  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* gdb.cp/step-and-next-inline.exp: Fix test case.