build: Fix double quotes in rules file

MBIM_USERNAME is quoted twice in the 76-mbim-proxy-device-ownership.rules rules file. This is because the definition is used in two files.

The first one is config.h file, that is used in source code files and needs to be explicitly quoted. The second one is the rules file, mentioned above that already has quotes in the template file.

The explicit quotes have been removed in the rules file generation.

Fixes #21 (closed)