meson: Replace 'config.h' with config_file

This fixes an issue with libdrm failing to build when used as a meson
subproject. Using 'config.h' directly will cause it to possibly refer to
the wrong file.

By using `@0@.format(config_file)`, it will be transformed into the
correct relative path, e.g. `./config.h` in normal build,
`./subprojects/libdrm/config.h` in subproject build.

Signed-off-by: Scott Anderson <scott@anderso.nz>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>