Add ugly cast for our siginfo_t forward declaration.

Add ugly cast for our siginfo_t forward declaration.

gcc complained:
/root/rpmbuild/BUILD/libi2ncommon-2.4/utils/signalfunc.cpp: In function ‘bool I2n::SystemTools::install_signal_handler(I2n::SystemTools::Signal, void (*)(int, siginfo*, void*))’:
/root/rpmbuild/BUILD/libi2ncommon-2.4/utils/signalfunc.cpp:370: error: invalid static_cast from type ‘void (*)(int, siginfo*, void*)’ to type ‘void (*)(int, siginfo_t*, void*)’

We can get away with casting function pointer types on x86:
http://stackoverflow.com/questions/559581/casting-a-function-pointer-to-another-type

Not sure if it's really worth to avoid including signal.h from signalfunc.hpp...