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 <[email protected]>

Author: ewalkusx

scripts/build-unit-test-docker

@@ -859,6 +859,7 @@ RUN apt-get update && apt-get dist-upgrade -yy && apt-get install -yy \
     libjson-perl \
     libldap2-dev \
     libmimetic-dev \
+    libmpfr-dev \
     libnl-3-dev \
     libnl-genl-3-dev \
     libpam0g-dev \