easydbus: fix build without C++

Specify that easydbus is a C project file otherwise build will fail if
no C++ compiler is found by cmake

Fixes:
 - http://autobuild.buildroot.org/results/486c3cd98124e7415dee2fd1463bd5e0fcc9ba91



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>