The libfprint library is generally built with C calling conventions.
Which makes it difficult to link to C++ programs as it is.
This patch adds the support for linking with C++ code - by telling the C++
compiler to use C calling conventions for libfprint functions.
Reported-by: Guus Ellenkamp <guus@activediscovery.net>
Signed-off-by: Kunal Gangakhedkar <kunal.gangakhedkar@gmail.com>