meson: Fix typo in gsm header file name
This was wrong since commit c360ceea.
Also fix incorrect indentation (tab instead of spaces).
Found using hermetic builds with Nix:
https://github.com/NixOS/nixpkgs/pull/54398#discussion_r280125735Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
This was wrong since commit c360ceea.
Also fix incorrect indentation (tab instead of spaces).
Found using hermetic builds with Nix:
https://github.com/NixOS/nixpkgs/pull/54398#discussion_r280125735