mutt: fix static build with libidn2 and libunistring

When libidn2 is statically build with libunistring support, mutt needs
to add -lunistring to LIBS.
To do that, add a call to PKG_CHECK_MODULES to retrieve this information
from libidn2.pc

Fixes:
 - http://autobuild.buildroot.net/results/177da8f4798f69298db5385957184f1c53cca923



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>