fix usage of broken feature in meson.build file

It seems that we are trying to convert file objects to strings which is
considered as a misfeature since it does not work reliably.

This is the warning that is seen while building pldm:

``` WARNING: Broken features used:
 * 1.3.0: {'str.format: Value other than strings, integers, bools, options,
   dictionaries and lists thereof.'}
```

Tested:

Rebuild pldm repository with the fix and it builds pldm without the
mentioned warning above.

Change-Id: If8fedf488d33de6dccc2c1b45c2bfe124fbc3384
Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>