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.