build-unit-test-docker:  Add libmpfr-dev

After meson bump, clang-tidy reports an error when using
boost::multiprecision based on the MPFR library

```
/usr/local/include/boost/multiprecision/mpfr.hpp:22:10: error: 'mpfr.h' file not found [clang-diagnostic-error]
```

Change-Id: I76db10af842926f5767c897b92f1ccc0f63ffdbe
Signed-off-by: Ewelina Walkusz <ewelinax.walkusz@intel.com>
1 file changed