scangobj: Avoid generating unused params
The main function params may be unused depending on type init function
definition. That may lead to generating a warning about unused in a
project using gtk-doc, eg:
DOC Scanning header files
DOC Introspecting gobjects
spice-gtk-scan.c: In function ‘main’:
spice-gtk-scan.c:126:11: warning: unused parameter ‘argc’ [-Wunused-parameter]
main (int argc, char *argv[])
^~~~
spice-gtk-scan.c:126:23: warning: unused parameter ‘argv’ [-Wunused-parameter]
main (int argc, char *argv[])
Check for the presence of argc and argv in the type init function
and generate main function params accordingly
https://bugzilla.gnome.org/show_bug.cgi?id=773879